Replace JavaScript

Hello. Faced a problem. I need to use a script to automatically change the text on the site to text which I will write in it This is not needed for my site, but to change the site code for yourself.For example using Tampermonkey.

Hello there!

It is not clear as to what you are trying to change.
Can you add a link to a repository or provide screenshots?
What exactly are you trying to change on github.com ?