How merge a master branch into some other branch in Web UI?

Previously I used Bitbucket for my projects. And Bitbucket’s web interface provides a nice ‘Sync’ option for branches. It merges master-branch (if it went forward) in any selected branch from the UI.

I have not found, is there a similar option in Github web interface?

I only know the other way: you can sync your PR branch with the main branch.

GitHub will do that automatically for your and it shows you if there are merge conflicts.

Do you mean synching when I merge PR back into main branch. But can I merge commits from the main branch into PR in progress?