GitHub Desktop 2.9 includes squashing, reordering, amending, and more!
The latest version of GitHub Desktop allows you to squash commits, squash and merge, reorder, amend your last commit, check out a branch from a previous commit, and more.

Source: The GitHub Blog
The latest version of GitHub Desktop allows you to squash commits, squash and merge, reorder, amend your last commit, check out a branch from a previous commit, and more.