If the commit is the head of current branch, that is easy.
- Context menu -> Git Commit
- Tick "Amend Last Commit" checkbox
- Correct your commit message
- OK
If the commit is the head of other branch, switch to that branch first.
- Context menu -> TortoiseGit -> Switch/Checkout
- Choose branch name
- OK
- Follow above 1-4 steps to amend commit message
If the commit is in the middle without any merge between head, you need to reset, amend and cherry-pick
- Context menu -> TortoiseGit -> Log
- Select the commit -> Context menu -> Reset
- Hard Reset (this will discard all work contained in commits above the selected commit as well as any un-committed changes in the working directory)
- OK
- Follow above 1-4 steps to amend commit message
- Select from head to one commit above it -> Context menu -> Cherry-pick
- Continue