Jerry has previously added me as a collaborator.
Not sure how to check, if you try to edit a file in Github what buttons do you see down the page? Here for example https://github.com/jerry3904/mxum/edit/master/README.md
If you see "Propose file change" it means you don't have edit rights. Otherwise you'll see "Commit changes". If you don't see "Commit changes" Jerry will need to add you as an editor to his repo.
With the second option, I am not seeing the provision to send a pull request to Jerry
Where is your repo on Github? https://github.com/jerry3904/mxum/network
To be able to do that you'd need to fork Jerry's repo, and pull and push to your repo. Then from Github in your repo you'd have a button to do pull requests.
So you'll have to have a Github account just like Jerry: https://github.com/jerry3904
(but of course with your user name) then once you are logged in you could fork projects from other people, then pull and push to your repo and when you are done with the changes issue Pull Requests.
Git and Github are not very simple, but with a bit of patience and solving problems step by step you should be able to start to use it. Once you get into a pattern you'll see it's not impossible but we need to go over this first time user obstacles...