GitExtensions doesn’t store any passwords.
They are stored by git or, more exactly, by “git credential manager” that stores them inside windows.
Search for Credential manager* in the start menu and you should find it. Otherwise, it should be in: Control panel => User Accounts => Credential manager
Then select the tab Windows credential and you will see keys named like git:xxxx.
Detailed steps in this answer :
https://stackoverflow.com/a/39608906/717372
*: what to look for on system in other languages:
- In french, “Gestionnaire d’informations d’identification”
- In German, “Netzwerkkennwörter verwalten”