Why would you feel the need to change the elapsed time for an activity anyway, when it is not used for calculating the summary statistics (e.g. average speed)? If elapsed time is noticeably longer than the ‘timer’ time for the activity record, then it simply means you left your recording device in paused/stopped mode for a while before you decided to terminate the session and save the activity. OK, in this case it's not exactly how long you left your Garmin device in activity mode, but I'm not sure in what way the elapsed time actually matters.
It seems there is no way to change the elapsed time.
The elapsed time matters because of uploading the activities to Strava. When I uploaded a bike ride that took 1 hour 20 minutes from Garmin Connect (elapsed time shown as over 60 hours), Strava showed the time as 4 hours so all the segments were messed up.
