It does not appear that feature is currently included in GitHub desktop. What I normally do is click on the repository name in the drop-down menu in the top left and then click on “open in terminal”. Then just follow the instructions here.
Update:
If you add this to your .gitconfig
, GitHub desktop should rebase according to here.
[pull]
rebase = true
Second Update:
GitHub desktop 2.0 now supports rebasing built in! It is under the branch section of the top menu or you can use the shortcut ⇧⌘E