In Garmin Connect, when you are on the page where you view the individual activity you can select the 'pencil' icon from the tool bar near the top of the page and do a 'full edit'. You can change the important numbers there.
If you want to edit or trim the end of the gps track, then you can upload to Strava and trim it with only a free account. After you trim it there, you can export it and then manually upload it to GC again. There are other sites that let you do this too, but I'm not sure of the others. I'm pretty sure you can trim it with software such as Garmin Training Center or Garmin Basecamp, but I've never tried trimming with them.
