You should be able to do this in the Library > Chord Symbols dialog. Type a suitable chord symbol that contains the appearance you want to change in the box at the top left of the dialog, and add it to the Project Default Appearances list – don’t worry, you can remove this again in a minute.
Now select the “mi” part of the chord symbol in the preview on the right. You should see a number of different appearances for that “mi” component appear in the grid below the preview, and the current one will be selected. Click the pencil icon in the action bar to edit that appearance. Make whatever changes you need to make, and OK the editor.
You can now remove the chord symbol you temporarily added to the list of Project Default Appearances, and confirm the parent dialog.
You should now find that whenever you create a chord symbol in this project that uses that “mi” appearance, it should automatically use your edited appearance instead of the default one.