Yes, you can change username of your git hub by following the steps.
click your profile picture in the top right corner, then click Settings. On the left side, click Account. Then click Change username.
But there are some effects while changing the username for more details go through this before changing username
https://help.github.com/en/articles/changing-your-github-username
For more information about installation of git and common git commands please go through the following link: