To change the commit message for commit abcdef1 in your Git repository, you can use the git rebase command with the interactive option. Here are the steps:

1. Start by creating a new branch to make the changes on:
```
git checkout -b fix-commit-message abcdef1
```

2. Then, use the interactive rebase option to modify the commit message:
```
git rebase -i abcdef1~1
```

3. This will open your default text editor with a list of the commits that are going to be rebased. You should see the commit abcdef1 at the top of the list. Change the first word of the commit message from "pick" to "reword", then save and close the file.
```
reword abcdef1 <commit message>
```

4. Git will now pause the rebasing process at the commit you want to modify. It will open another editor window to allow you to modify the commit message.

5. Git will now continue with the rebase process. If there are any conflicts, resolve them as you would during a regular rebase.

6. Once the rebase is complete, you can delete the temporary branch:
```
git branch -d fix-commit-message
```

7. Since the two branches (12-fix-a and 13-fix-b) depend on commit abcdef1, they will need to be rebased on top of the updated commit. First, checkout `12-fix-a`:
```
git checkout 12-fix-a
```

8. Rebase the branch onto the updated commit:
```
git rebase fix-commit-message
```

9. Repeat the previous two steps for `13-fix-b`

This will update the commit message for commit `abcdef1` and preserve the commit structure, while also ensuring that the dependent branches are rebased on top of the updated commit.
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