To apply a source modification to your add-on from GitHub, you need to perform the following steps:

1. Stop the instance using that add-on.
2. Make a backup if it is a production instance (refer to the Manage Odoo server databases recipe in Chapter 1, Installing the Odoo Development Environment).
3. Go to the directory where partner-contact was cloned:

$ cd ~/odoo-dev/my-odoo/src/partner-contact

4. Create a local tag for the project so that you can revert to that version in case things break:

$ git checkout 12.0
$ git tag 12.0-before-update-$(date --iso)

5. Get the latest version of the source code:

$ git pull --ff-only

6. Update the partner_address_street3 add-on in your databases (refer to the Installing and upgrading local add-on modules recipe).
7. Restart the instance.