If a feature branch is behind master, you can sync that branch, using a merge, into your feature branch.

1. (On macOS) From the left-side menu, hover your mouse over the right side of the Branches label until you see the word Show. Click Show to expand the list of branches.
   
   
2. Under Branches, double-click the feature branch that is behind to switch to that branch.
3. Click the Merge button.
4. From the popup that appears, select the commit you want to merge into your feature branch.
   
5. Check the Create a commit even if merge resolved via fast-forward option at the bottom.
6. Click OK.
   Your feature branch now has the same commits as your master branch.