You can use rebase to do this.
It looks like the Windows version does not support interactive rebasing (https://jira.atlassian.com/browse/SRCTREEWIN-405) yet. I don't know of another way to do this in SourceTree, but maybe someone else will.
You could also try to rebase with the command line - http://stackoverflow.com/a/2719636/1133218
Looks like you can do it, at least in version 220.127.116.11 on Windows: Stage (add) your changes, as if you were going to perform a new commit. Then under "File Status" put your cursor into the Commit message box to open up the full commit pane. Top right choose from "commit options..." and select "Amend latest commit". You then get to copy the message from the previous commit into the message box, edit it if you want and commit. It performs a `git commit --amend` for you and voila - the previous commit is amended. Loverly.
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