How to manually (down) correct MPA and FTP?

I had big trouble with my powerpedals (powertap) the last couple of weeks because erratic data/calibration on one of the pedal : then the recorded power by my garmin devices are way too high. I got a MPA above 1100W (2 times what I got previously) and the average power is also +50% too high giving a FTP +50% what I really are.
How to manually (down) correct this erroneous MPA and FTP? (I don’t want to delete the related workouts as other parameters [HR, speed, cadence, distance …] except power are ok). Is the only way to correct MPA and FTP is to delete the erroneous workouts then remove the (erroneous) power values from the garmin .Fit files and reimport the excluded power files?

There are no direct ways to adjust power data in an activity file. You can manually assign and lock a signature to an activity. This often suffices to enable you to keep the activity but not have if affect your progression.

if you have a fit file try to make adjustments then upload to Xert