After changing my password on the remote, I could not get Sourcetree to log in no matter what. I tried editing and deleting my accounts, deleting and re-adding the repo, nothing was working – Authentication failed every time.
Finally this solution worked for me:
navigating to C:\Users\USERNAME\AppData\Local\Atlassian\SourceTree and removing (or renaming) the passwd file.
Once this file is removed, restart SourceTree and execute a fetch or something else that requires access to the repo in question. SourceTree will then prompt you for your password, rewriting the cached credentials.