You can provide a name for the stash in the dialog box that opens when you create a stash:
- Stage some changed files
- Click the "Stash" button in the toolbar
- The following dialog shows up. Provide a name in the "Message" field.
- The provided "Message" will be used as the name of the stash

If you are trying to rename an existing stash, it looks like there's no way to currently do this in SourceTree - the only options in SourceTree are to apply or to delete the selected stash.
Git in general does not provide a built-in way to rename a stash - the consensus seems to be to drop and recreate the stash with a new name. See how can I rename a git stash for a question that addresses this in more detail.