If you're looking at the tree, with the graph view, find the revision you want to check out, right-click on it, and select Update...
But let's say that you're having trouble finding the commit -- maybe you have a lot of branches, and so you don't see the revision you want to check out. Turn on the Filter Toolbar (View > Filter Toolbar).
Look on the right. You'll see a dropdown that says something like ★ Show all ★
. Click on that to list all your open branches. Select the branch you want to check out, and it will filter the graph view so only commits on that branch show up. Then it'll be easy to find the most recent commit on the default
branch, because it'll be the top one.