According to Garmin:

Delete a PR

1. Sign in to Garmin Connect web from a web browser.
2. Select the Arrow Icon in the upper left corner to expand the navigation bar (if it is not already expanded).
3. Select Personal Records.
4. Select the activity type (example: Running).
5. Select. ...
6. Select Remove to confirm.

Amazing, thank you! I always forget there's a website.

It annoys me that there are things you need the website for that you can't do on the app. And you can't open the website on a device with app installed without it default opening the app.