Here’s how to do it:
- Open Sourcetree and go to the repository.
- In the history view, find the commit you want to squash into. Right-click one commit before the oldest one you want to squash into and choose 'Rebase children of <commit> interactively…'
- The 'Interactive rebase' window will open. Leave the top commit and change the next ones.
- Click 'OK' to finish the squash.
- Sourcetree will perform the rebase and squash the commits.