I have found a serious computational mistake I made after I passed my viva and submitted (finally) my thesis to the university where I studied, what should I do about it?

I am really very pissed about it, the mistake is serious but not fundamental to my thesis (e.g., the conclusion is my method works, with or without such mistakes I made), what should I do about it? need some suggestions from you.

I am really quite pissed about that, the computational mistake make my thesis looks like a pile of crap :-(

Is this possible to ask the examiners or college staff to update my thesis after final submission?

I really need some suggestions!:-s

No idea. Ask your supervisor?