The distances cannot be entered or edited for the club list in the "My Bag" menu.
Distances attributed to each club can only be derived from GPS distances tracked using the AutoShot feature that are recorded through your scorecards.
If you have max distances or outlier shots affecting the average distance values for any of your clubs, the shot must be corrected on hole it was recorded from through your scorecards. More info in correcting club stats outlined on the Garmin Support page linked here: Correcting Golf Club Stats in the Garmin Golf App and Garmin Connect
Looking all home of all score card on the mobile app is si time consuming. I got many wrong recording (most of the time because i made mistake like forget to change the hole on course that have been changed or other reason). To find easily the mistake I've been on the garmin website and requested all my data from garmin. With this data you can have your data in json file easily readable on the web.
That so bad garmin have the data but dont make a good use of them
