You can update Modmail on your Heroku account whenever changes are made to the repository.
Fork the repo by clicking the Fork button at the top right of this page (must be on desktop site). Then on your Heroku account, go to the modmail application. Click on the Deploy tab and connect the repo named yourusername/modmail. Do a manual deploy of the master branch.
Click here to create a new pull request to your fork. Select compare across forks, make the base repository yourusername/modmail and ensure the branch is set to master. Put any title you want and create the pull request. On the page that comes after this, merge the pull request.
You then want to go to your modmail application in Heroku, connect your modmail fork via the Deploy tab and deploy the master branch.
You can turn on auto-deploy for the master branch if you don't want to go through the process of logging into Heroku and deploying the branch every time changes to the repo are made in the future. However, you will have to make a pull request to update your fork everytime.
If you haven't done so, make sure to make a fork of the Modmail repo. Click the Fork button at the top right of this page (must be on desktop site).
You can register your fork of the repository to the Pull app, which will check for updates and make those to your copy of the code. Click the Install button. A new screen will pop up. Check the Only select repositories option and select yourname/modmail. Click the Install button and enter your password if needed. It will give you a confirmation message and bring you to the settings tab, where you can delete the app if you don't like it. Turn on auto-deploy for the master branch in Heroku, this will deploy the branch every time changes to the repo are made in the future.