Can answer myself, have looked in the instructions online:
The clinical thermometer contains a long-life alkaline manganese battery (LR 41, 1.5 V) for approx. 2 years. The battery must be changed when "T" appears at the bottom right-hand edge. is displayed. Press the battery compartment cover at the end of the device with your fingers and pull it firmly backwards. backwards. Remove the old battery and replace it replace it with a battery of the same type.