What should I do, as I discovered that my wife is not a virgin, and after concealing her situation, it became clear that she is pregnant by someone other than me?

The disappearance of the hymen is not proof of a woman's lack of chastity. The basic principle is that the pregnancy that became apparent from you is your child, for the child belongs to the bed (the husband) if born within the period of possibility (six months). However, if you are certain that it is not from you, you may disavow it by performing mula'anah (oath of condemnation) with your wife, which leads to permanent separation and the denial of the child's lineage to you.