Let’s say there is your-branch and the master branch. You want to merge changes from your-branch into the master for others to see them, but someone else did conflicting changes to the master (e.g. merging their PR) in the meantime. It is often useful to merge master into your-branch (i.e. do the merge the other way round) before creating the PR.
In the command line, you can:
git checkout master
git pull
git checkout your-branch
git merge master
Now you can see the list of conflicts. Follow the messages you get from git to resolve the conflicts. You can use your favorite tools, so it is way easier. Finally, you commit&push. When you re-create the PR, there should be no conflicts.