Comment by esperent

If the code is in a repo that has been shared with other people and with a license file saying "All code in this repo is under MIT license", then the code is MIT licensed. You can't come along later and say "I changed my mind" or "I made a mistake". Hard luck, be more careful next time.

The only exception would be if you ask the people whom it was shared with if they're ok with retroactively changing the license.