Sourcetree change password of existing account

Solution 1:

I had to manually clear stored credentials by emptying those files on my Windows:

%LocalAppData%\Atlassian\SourceTree\userhost
%LocalAppData%\Atlassian\SourceTree\passwd

Then restart SourceTree.

Solution 2:

On OS X, if using a company hosted git repo, and you changed the password due to company password expiration policy, you need to go to keychain access, search for sourcetree in there. An application password will apear for SourceTree. Delete it. Next time you try to fetch/push/whatever on your repo, it will ask for pass and will create a new entry in keychain.