1) Export the activity from the GC website 2) Use Fit File Tools (section remover) to crop the run: https://www.fitfiletools.com 3) Delete the original activityfrom GC and import the edited version
Some people who sync with Strava like to crop in Strava, which is easy enough to do. I'm not sure if I like that option (if you need to re-import to GC) because I think the Strava might not preserve all the data from the original file.
You can also use Garmin Basecamp to trim the GPX file, the re-import to Garmin Connect. You must remember to delete the activity from Garmin Connect else it will reject the import advising this activity already exists.
guys there is now a solution in connect.garmin.com simply choose the activity, hit edit (top right corner) and scroll down to trim activity. There is a scroll bar that you use to trim beginning and end!