Unable to change Git account

This has nothing to do with your user.name/user.email settings: those are for authorship in a commit. They are not used for authentication when you push to a repo.

If Git does not ask you for your GitHub (new) username/password, that means Git for Windows is using a Git credential helper called "manager" (do a git config credential.helper to confirm it)

Meaning: it is caching your old credentials and is reusing them automatically.

In that case, go to the Windows start menu (), type "credential" and select the Windows tool "Windows Credential Manager".

In it, you will find an entry git.https://github.com, which you can edit, and where you can enter your new GitHub username/password.

Then try and push again.

With more recent Git version (2.32+, Q2 2021), assuming <C:\path\to\git>\usr\bin and <C:\path\to\git>\mingw64\libexec\git-core are in your %PATH%, you can do the same removal in command-line:

printf "protocol=https\nhost=github.com\nusername=xxx"| git-credential-manager-core erase