What is the ruling on keeping a theatrical award that the questioner participated in, and now regrets participating in because it contained unlawful matters, while he is unsure if he knew about this beforehand? And how does one repent from mocking something from the religion or the hadith and then regretting it?

If the play is beneficial and permissible, there is no harm in accepting the prize, especially if you were unaware of the impermissibility of what it contained. As for the repentance of one who mocks religion: if his mockery does not amount to apostasy, then repentance is by regretting and resolving not to return to it. But if it was apostasy that expels one from the faith, then his repentance is accepted between him and Allah, and it is by uttering the two testimonies of faith (Shahadatayn) and retracting what he believed if his apostasy was due to denying an obligation or deeming a forbidden act permissible.