Cannot calibrate indoor run for Apple Watch 6
There is no manual calibration. You can hope that your indoor stride matches your outdoor stride and use it to refine the step lengths.

https://support.apple.com/en-us/HT204516

The system calculates stride length based on GPS and pace, so if you’re is off, resetting the existing calibration and repeating the outdoor calibration in straight line / flat conditions might be worthwhile.