Then declare the perfection of your Master and pray to him as thanking for His blessings upon, and seek His forgiveness for you and for your followers.
Asking forgiveness of Allah (swt) from the side of the Prophet (saws) was because of not doing the best, not because of any sin or guilt he (saws) has committed.
Sayedah A’isha (may Allah be pleased with her) said:
The Messenger of Allah (saws) recited often these words: Hallowed be Allah and with His praise, I seek the forgiveness of Allah and return to Him. She said: I asked: Messenger of Allah, I see that you often repeat the saying” subhan allahi bihamdihi astag firullahi watubuilaih” whereupon he said: My Lord informed me that I would soon see a sign in my Ummah, so when I see it I often recite (these) words: Hallowed be Allah and with His Praise, I seek forgiveness of Allah and return to Him. Indeed I saw it (when this verse) was revealed:” When Allah’s help and victory came, it marked the victory of Mecca, and you see people entering into Allah’s religion in troops, celebrate the praise of Thy Lord and ask His forgiveness. Surely He is ever returning to Mercy.”
Exaltation means not attributing any characteristics or notions that are not befitting to the divine nature of Allah (swt). Examples of the verses that contain the exaltation:
﴾سُبْحَانَ الَّذِي أَسْرَىٰ بِعَبْدِهِ﴿
(Exalted is He who took His Servant…) (Al-Issra: 1)
﴾سُبْحَانَ الَّذِي خَلَقَ الْأَزْوَاجَ كُلَّهَا مِمَّا تُنبِتُ الْأَرْضُ وَمِنْ أَنفُسِهِمْ وَمِمَّا لَا يَعْلَمُونَ ﴿
(Exalted is He who created all pairs – from what the earth grows and from themselves and from that which they do not know.) (Yaseen: 36)
﴾فَسُبْحَانَ اللَّهِ حِينَ تُمْسُونَ وَحِينَ تُصْبِحُونَ ﴿
(So exalted is Allah when you reach the evening and when you reach the morning.) (Ar-Room: 17)
﴾سَبِّحِ اسْمَ رَبِّكَ الْأَعْلَى ﴿
(Exalt the name of your Lord, the Most High,) (Al-A’laa: 1)
So, Exalting of Allah (swt) means to regard Allah as pure and free from every blemish.
بِحَمْدِ رَبِّكَ (with praise of your Lord): The Arabic word “praise” is infinitive, meaning, to praise your Lord in a decent way. It is Allah’s favour on His servants that He (swt) taught them how to praise Him (swt) by saying (Al-Hamdullellah) (All Praise to Allah). Thus, Allah (swt) did not leave it to our various methods and languages to decide the ways of praising. So (Then exalt [Him] with praise of your Lord) means to praise Him (swt) in the way He (swt) taught us. Due to this, the Prophet (saws) was saying in his supplication:
“I cannot count Your praises. You are as You have praised Yourself”
Or it may mean, say: (Subhanallah Wa Behamdehi) or (Subhanallah Wa Al-Hamdullillah).
وَاسْتَغْفِرْهُ ۚ إِنَّهُ كَانَ تَوَّابًا (and ask forgiveness of Him. Indeed, He is ever Accepting of repentance): If the Holy Qur’an had been from other than Allah (swt), it would have been written in a different manner.
Linguistically, the explanation should be suitable to the command, that is to be (and ask forgiveness of Him, indeed He is ever the Forgiver) or (and repent to Him, indeed He is ever Accepting repentance).
But the Quranic composition is other than that, that is to bring two matters, each of them has two elements, in this case we have four elements, two for each matter of them. In other words, (and ask forgiveness of Him. Indeed, He is ever Accepting of repentance) means (and ask forgiveness of Him. Indeed, He is ever forgiver and repent to Him.
Indeed, He is ever Accepting of repentance). Thus, one statement concludes two matters: The repentance and the ask for forgiveness.
What is the difference between to ask for forgiveness and to ask for repentance? Asking for forgiveness is done after committing a sin while asking for repentance means to return back to the divine method of Allah (swt) with the intention of never going back to the sin.
The speech is directed to the Prophet (saws) which means it is directed to the whole nation as well. That is why we find Allah (swt) saying in some verses:
﴾ إِنَّ اللَّهَ بِمَا تَعْمَلُونَ بَصِيرٌ ﴿
(Indeed, Allah of what you do, is Seeing.) (Al-Baqara: 110)
The Arabic word (you) is in plural form not in singular.
This is the apparent meaning of Surah Al-Nasr which every reciter of it may understand whereas, there are meanings beyond the words have to be pondered upon and concluded.
﴾أَفَلَا يَتَدَبَّرُونَ الْقُرْآنَ ﴿
(Then do they not reflect upon the Qur’an?) (An-Nissa: 82)
We should not stop at the apparent meaning, rather we have to search for what behind it. Thus, Allah (swt) states to us many matters, additionally, gives us a great favour through teaching us how to praise Him and how to ask for forgiveness and repentance.
(Ibn ‘Abbas (May Allah be pleased with them) said:
‘Umar (May Allah be pleased with him) used to make me sit with the noble elderly men who had participated in the battle of Badr. Some of them disliked it and said to ‘Umar: “Why do you bring in this boy to sit with us when we have sons like him?” ‘Umar replied: “Because of the status he has, which you already know about (i.e., belongs to the source of knowledge and the house of the Prophet (ﷺ)).” One day, ‘Umar called me and seated me in the gathering of those people; and I think that he called me just to show them (of my religious knowledge). ‘Umar then questioned them (in my presence). “How do you interpret the ayah of Allah: ‘When there comes the Help of Allah (to you, O Muhammad (ﷺ) against your enemies) and the Conquest (of Makkah).”‘ Someone said that when Allah’s Help and the Conquest (of Makkah) came to us, we were called upon to celebrate the Praise of Allah and ask for His forgiveness. Some others remained silent and did not utter a word. Thereupon ‘Umar asked me: “Ibn ‘Abbas! Do you say the same.” I replied: “No”. He said: “What do you say then?” I replied: “That is the sign of the Prophet’s death about which he had been informed. Allah, the Exalted, says: ‘When there comes the help of Allah (to you, O Muhammad (saws) against your enemies) and the Conquest (of Makkah)’. So declare the remoteness of your Rubb from every imperfection, and ask for His forgiveness. Verily, He is the One Who accepts the repentance and Who forgives”. On that ‘Umar (May Allah be pleased with him) said: “I do not know anything about it other than what you have said”.
 Sahih Al-Bukhari, Book of Al-Maghazi, (Military Expeditions led by the Prophet (saws), hadith No. 4043.