Impressed to see a response to a post that is over 3 years old!
I don't think the original saved activity file is change if you edit the activity on Garmin Connect interweb so even sending to device probably won't work. Try it and see. You get the menu shown below by clicking on the small cog in the top right of an opened activity in GC interweb.
The other way to achieve you objective is to use Fit File Tools change the distance there, then download the file to your computer desktop. Connect your watch to your computer with the USB cable and open the Garmin folder. In there you'll see the Activity folder that has all your recorded activities. The file names are date stamped but using a hexadecimal code so unless you can read hex you'll need to order by date created and identify the one you want to replace. Rename your downloaded file then place it in the Activity directory overwriting the original.
But even if you can be bothered to do all that, pretty sure it won't change the totals on the watch.
