Praise be to Allah.
The final tashahhud is a pillar or essential part of the prayer, and the first tahahhud is an obligatory part, as we have stated in the answers to questions no. 65847 and 125897.
The Muslim should adhere to the wording of the adhkaar that have been prescribed for the prayer and other matters, and should refrain from changing any part of them as much as possible.
Al-Bukhaari (6265) and Muslim (402) narrated that Ibn Mas‘ood said: The Messenger of Allah (blessings and peace of Allah be upon him) taught me the Tashahhud, with my hand between his, as he taught me a soorah of the Qur’an.
What this means is that we should pay great attention to the wording of the tashahhud and not add anything to it, take anything away from it or change any part of it, paying as much attention to this as to the Holy Qur’an.
Based on that, it is not permissible for the worshipper to say, “As-salaamu ‘alayka wa ‘ala ‘ibaad-illah as-saaliheen (Peace be upon you and upon the righteous slaves of Allah)” , instead of “As-salaamu ‘alayna wa ‘ala ‘ibaad-illah as-saaliheen (Peace be upon us and upon the righteous slaves of Allah),” because this is changing the wording taught by the Prophet (blessings and peace of Allah be upon him), and because it is also changing the meaning.
Our advice to you is to adhere to the wording that was narrated from the Prophet (blessings and peace of Allah be upon him). The best book – as far as we know – that describes the prayer and the wording of the du‘aa’s and dhikr to be recited in the prayer is Sifat Salaat an-Nabi (sall-Allahu ‘alayhi a sallam) [English title: The Prophet’s Prayer Described] by Shaykh al-Albaani (may Allah have mercy on him), so you should acquire a copy, read it and act upon what it says.
And Allah knows best.