How to fix the distance of a treadmill run

On a treadmill there's no GPS, so your watch estimates distance from your wrist or a foot pod. It's often off by a few percent, sometimes by much more, and the error carries into your pace, your splits, and anything a platform works out from them. The treadmill's own display is usually closer. The fix is to put that number in the file.

Fix it on the watch, if you can

Many Garmin watches can calibrate the distance when you save a treadmill run of at least 1.5 km (1 mile): choose Save the first time, or Calibrate & Save later, and type in the distance from the treadmill's display. That corrects this run and helps the next ones. Button names differ by model, so check your watch's manual under "Calibrating the Treadmill Distance".

If the run is already saved and synced, fix the file instead.

Fix it in the file

Get the .fit file. From Garmin Connect, open the activity, gear icon, Export Original (it downloads a zip; open it as it is). From Strava, three-dot menu, Export Original.
Open it in FIT File Editor.
Open the details panel next to the file name and type the real total in Total distance.
Press Export.

Every distance in the file scales by the same amount: the running total on each data point, each lap, and the session. Speed and pace follow, so a 5:30 /km run that was really 6% longer reads as about 5:11 /km. Time, heart rate, and cadence don't change.

The edit applies to the whole run. Every lap scales by the same amount, so if one lap was badly off and the rest were right, the total comes out right but the laps don't.

Other things to fix while it's open

Logged as an outdoor run. Set the sport to Running and the type to Treadmill in the details panel. See change the sport.
A warm-up walk or a stop at the end. Drag across it and press Cut. See crop a FIT file.
Heart rate spikes from a dry strap at the start. See the heart rate spike guide.

Upload it

Delete the old copy first, then upload the edited file. Platforms reject a file that overlaps an activity they already have. The duplicate activity guide has the order for Strava and Garmin Connect.

The same fix works for

An indoor ride where the trainer or app reported the wrong distance.
A wrong wheel size on a bike computer with a speed sensor: every ride reads long or short by the same percentage. Fix the setting on the unit for next time.