I'm going to assume for the moment that you're using a credential helper. This is the reasonable and secure thing to do.
The easiest way to reset any credential helpers is to tell the credential helper that the password is bad and should be rejected. Normally this occurs when your password is bad anyway, but sometimes it doesn't. You can do this with the following (substituting origin if necessary):
$ echo "url=$(git remote get-url origin)" | git credential reject
If that doesn't work, you can also try to give the credential helper a hint by adding a username. For example, if I were trying to reset my GitHub credentials, I'd write the following:
$ echo url=https://[email protected] | git credential reject
Usually that should be sufficient. It is possible if you're using one of the Microsoft-built credential helpers that they don't honor that setting, but most of the standard ones on Linux should.
Note that there is no user.password config setting. The user group of configuration options is not used for remote servers, a secure configuration results in different passwords for different servers (which the hypothetical user.password would not), and storing secrets in the .git/config is not secure.
If none of these work, you'd need to inspect your Git configuration with git config -l. You'd be looking for a credential helper (credential.helper or credential.*.helper) or some other configuration that contains credentials. Since you haven't included that, we can't really tell you what might be going on.
git config --global --unset credential.helpernewPasswith your own password of course. Otherwise usegit config --global --unset user.passwordto remove the users password (not from the helper)