Stored separately from the firmware itself, in the mainboard's EEPROM, you have statistics of your printer, calibration data related to the movement of the printer, and PID values for heating. If these values become corrupted or are not compatible with a new version of the firmware, you can have some issues. If so, we recommend doing a factory reset to clear these values.
Doing a factory reset will clear statistics data, like total printing time and filament use.
How to
Follow the article for Factory Reset (MK4/S, MK3.9/S, MK3.5/S, MINI/+, XL).