Is the salary earned from a job obtained through cheating on an exam considered unlawful (haram)? Is resignation from it obligatory? And how can one atone for this sin?

The exam papers should have been rejected because accepting them would have been assisting in betrayal, and Allah has forbidden assisting in sin. What happened is considered partial cheating, and repentance is obligatory. The salary earned is permissible as long as the work is performed efficiently.