Warning: Brand new SourceTree user, and inexperienced GitHub user!
I forked someone else's repo on GitHub a few weeks ago, and have made some changes to my fork.
However, since then, commits have been made to the master repo, and I now want to pull those changes to my fork. However, I cant seem to work out how to do this.
I had a look at the docs, but what they show bears little relationship to what I see. Don't know if that's because I'm on Windows and that page mentions MacOS...

Anyway, I clicked on Working Copy, as that's my fork, which is behind the master,and then clicked the Merge button. This gave me something that looked like in the docs. However, the commit graph (which looked the same as the bit you can see above) only showed commits on the master repo up to the point where I last got a copy of the files. None of the commits since then show up.
I tried clicking the OK button to see if it would pull anything from the master, but nothing happened.
Anyone able to help me? I need to get my fork up to date with the commits that have been made to the master repo since I last got the files.
Thanks