The Journal shows a compact history (Log) for the current branch:

Depending on the type of commit(s) selected, you can invoke various operations from the context menu, most notably, you can easily rewrite the history:

The behavior of how commit times will (or will not) be adjusted can be configured by system properties (smartgit.pushableCommits.preserveAuthorDate).

To just change the commit message of the last commit (even for a merge commit or if the working copy is not clean), invoke Local|Edit Last Commit Message.