How do you fix a mira sport electric shower if the overheat protection thermostat has tripped?

First you must check why the overheat (one shot) thermostat has operated

Is yours a hard water area ? If so the heater tank and heating element could be scaled up. This will severely limit the transfer of heat into the flowing water and cause an overheat of the heating element. The overheat will open the one shot thermostat.

Check the heater tank and if not scaled fit a new one shot thermostat. Obtainable from Mira as a spare part. If the heater tank is heavily scaled I also advise to fit a new heater tank