Stryd is designed to be accurate out of the box without calibration for most runners, but there are exceptions. If you ran a marathon (or other race) and found that Stryd did not measure the exact distance, these steps will help avoid future problems.
Check the watch settings and Stryd attachment
Compare Stryd with watch data. This is a complicated step, so submit a support request if you need assistance.
Did you use different shoes during the event? Stryd will be consistent between shoes if Stryd is in exactly the same position on the foot regardless of shoes. Some runners may experience that Stryd can be inconsistent with the current firmware if Stryd is not in the same spot.
Finetuning Stryd pace/distance to avoid future issues:
Head to the track. Run with your race shoes at race pace. The detailed procedure: Distance and Calibration
No access to a track? Use the Garmin Autocalibration feature.
You can use the Stryd measured distance to get an estimate of your calibration factor: 42.2 km / Stryd distance * 100