What should I do if I find a mistake in my submitted master's thesis?
Everyone makes small mistakes from time to time and it is expected that there should be some mistakes in a master's thesis. What you should perhaps do depends on the nature of the mistake. If you are talking about a small or even a big typo, I would suggest just leaving it be and not alerting your professor. Typos are expected and a few won't impact your grade very much, if at all. However, if you have made a major scientific, analytic, or other substantive mistake, acknowledging it and offering a correction is the right thing to do. Depending on the rules of your institution, the professor may not be able to accept a modification to the document you turned in, but if they can, you are sorted. If they can't, them knowing that YOU know that this was a mistake is likely better than them finding the mistake and thinking that you don't know, from a grade point of view.