To apply and try out a GitHub pull request for an add-on, you need to perform the following steps:

1. Stop the instance.
2. Go to the directory where partner-contact was cloned:

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

3. 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)

4. Pull the branch of the pull request. The easiest way to do this is by using the number of the PR, which should have been communicated to you by the developer. In our example, this is pull request number 123:

$ git pull origin pull/123/head

5. Update the partner_address_street3 add-on module in your database and restart the instance (refer to the Installing and upgrading local add-on modules recipe if you don't know how to update the module).
6. Test the update—try to reproduce your issue, or try out the feature you wanted.

If this doesn't work, comment on the PR page of GitHub, explaining what you did and what didn't work so that the developer can update the PR.

If it works, say so on the PR page too; this is an essential part of the PR validation process, and it will speed up the merging in the main branch.