For just one repo, go into to the relevant repo DIR and:
git config user.name "Your Name Here"
git config user.email your@email.example
For (global) default email (which is configured in your ~/.gitconfig):
git config --global user.name "Your Name Here"
git config --global user.email your@email.example
You can check your Git settings with:
git config user.name && git config user.email
If you are in a specific repo which you setup a new user/config for (different to global) then it should show that local config, otherwise it will show your global config.