Merge/update a forked git repository

First add the "upstream" remote:

git remote add upstream remote_repo

Then fetch it:

git fetch upstream

Merge it with your local code:

git merge upstream/main

Resolve any possible conflicts (i.e. go through each file and edit it as necessary), commit and push them, and you're done!

Hopefully github's network graph will show you a nicely updated graph in your timeline ;)

Git network graph