With SourceTree on MacOS: Settings -> Advanced

Firstly, you need to choose which host you want to refactor:

• If you need to change both the username and password, please edit the username first. SourceTree will require the password when you interact with the Git repository.

• If you need to change the password only, I think it is better to delete the credential. When you interact with the Git repository, SourceTree will require you to enter a new one.