In a git repository I'd like to change the commit message for commit abcdef1. Two branches depend on this commit, including 12-fix-a and 13-fix-b. There are also several merge commits after commit abcdef1 and I want to keep that structure, so I don't think a simple rebase will do. How can I go about this?
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.
What if I want to use github desktop instead of command line?
If you prefer to use the GitHub Desktop interface rather than the command line, no worries! You can still achieve the same result by doing the following:

1. Open GitHub Desktop and navigate to the repository you want to modify.
2. Create a new branch to work on by clicking on the Current Branch dropdown menu and selecting New Branch. Choose a name that makes sense, like fix-commit-message.
3. Switch to the new branch by selecting it from the same dropdown menu.
4. Find the commit you want to modify in the commit history and right-click on it. Then select Edit Commit from the context menu.
5. In the dialog box that appears, simply modify the commit message and click on Amend Commit.
6. Once the commit has been amended, you can push the changes to the remote repository by clicking on Push origin in the top-right corner of the GitHub Desktop window.
7. Since the two branches (`12-fix-a` and `13-fix-b`) depend on commit `abcdef1`, you'll need to rebase them on top of the updated commit. Start by switching back to the master branch by selecting it from the dropdown menu.
8. Then checkout the `12-fix-a` branch by clicking on it in the Branches tab.
9. Click on the Update from master button to fetch any changes from the master branch.
10. Finally, click on the Merge into current branch button and select the fix-commit-message branch to merge the changes.
11. Repeat the previous two steps for `13-fix-b`.