Whilst adding a repository alongside a new project is easy with the OAuth method, you may wish to update an existing project's repository settings rather than create a new project, especially if you wish to keep the previous deployment history.
You'll need to firstly navigate to the project's main General Settings page, found via the Settings dropdown:
Next head to the bottom of the page, and copy the project's public (Deployment) key:
Now head to your repository host, and add the key to your repository's Deployment keys. Further information on updating this on your chosen host can be found below:
You'll now need to copy the SSH URL of the repository from the host as explained in the appropriate article above, and replace the Full URL to repository In Deploy:
Then just click Save Changes.
You may find you run into an issue when you next come to deploy the updated project; this will relate to a mismatch in commit references between the new and old repository. In this instance you'll just need to click the (change) link next to the start commit:
At the bottom of the commit selector, check the option to Deploy the entire Repository, then click Select Commit:
You'll now see your start commit populated with Beginning of time... - just click Deploy to finish.