I have changed my user and email for git with those commands
git config --global user.name "maa"
git config user.email "email@example.com"
git config --global user.name
git config user.email
git push -u origin master
remote: Permission to maa/brain.git denied to old_user_name.
fatal: unable to access 'https://github.com/maa/brain.git/': The requested URL returned error: 403
user.email settings are only used for commit meta data.
As I can see you are using https transport, so the username is stored somehow. To my mind there are two options:
https://firstname.lastname@example.org/...), here you need to update
.git/config, e.g. by issuing
git remote set-url https://<newusername>@github.com/<newusername>/yourRepo
As a general rule you can try the following on the CLI (based on 2), where you replace
$credentialhelper with your credential-helper (you can find it using
git config credential.helper, for Mac OS X it is likely
osxkeychain on Windows it might be
$ git credential-$credentialhelper erase host=github.com protocol=https [Press Return]