Might be easier for them to do it through the admin ui located at
- Change temporarily to another theme
- Delete old version of theme by clicking on it and then clicking on
deletein the bottom right corner of the pop-up/modal
Add Newbutton at top
- Select and upload the .zip file they downloaded from GitHub (or other location)
- Activate the updated theme
Fortunately needing to update themes doesn’t happen often. If you’re using a GitHub theme then be sure to “watch” the repository on GitHub and enable email notifications for it so that you’ll see any future updates, issues, or ongoing work to know about needing to update in the future.
Hint: this workflow could also be used to upload the theme from an external source in the first place.