I have been working in a government office and I used to be corrupt and I built houses with that money. But now I realise that what I used to do was not Islamic and I repent to Allah. And I am sponsored by the same office for a master's program and I have to go back to work for the same office. And I don't want to do what I used to do so can you please advice me what to do in order to be forgiven by Allah for my previous sins and not to do it again?

Praise be to Allah, the Lord of the World; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

You have to give back all the government money you had taken it, for the Hadith of the Prophet (Sallallahu Alaihi wa Sallam) that stated: "A hand (i.e. A person) is held responsible for what he had taken till he gives it back" . [Imam al-Tirmizi and graded it as Hasan Sahih].
You must feel sorrow for the past occurrences; ask forgiveness of Allah, strongly determine not to commit such a sin again; without the above matters your repentance is no longer valid. If you fear any harm in returning the money, then you may use any suitable safe way to give it back such as sending by draft without mentioning your name, etc.
If that is impossible, then you can spend the money for public benefit since all the government money is actually for public benefit. So, spending it in that field is similar to returning it to government.
Furthermore, spending such money for public benefit becomes compulsory if there is misuse of the public money in the governmental organizations. If you return the money to such organizations someone may use it unlawfully.
Allah knows best.