If you are a Windows user, you may either remove or update your credentials in Credential Manager.
In Windows 10, go to the below path:
Control Panel → All Control Panel Items → Credential Manager
Or search for "credential manager" in your "Search Windows" section in the Start menu.
Then from the Credential Manager, select "Windows Credentials".
Credential Manager will show many items including your outlook and GitHub repository under "Generic credentials"
You click on the drop down arrow on the right side of your Git: and it will show options to edit and remove. If you remove, the credential popup will come next time when you fetch or pull. Or you can directly edit the credentials there.