I have an old Lidl cycle computer and the spanner means it wants a service (when you initially set it up you tell it whether you want the service warning at 400, 600 or 800 miles).
For my computer you press the left hand button once which removes the spanner; it's the same button you use to reset the daily mileage, if that helps.
To change the clock on mine I just scroll round to the clock screen, press the left (reset) button for a few seconds, then I can do it.
Always worth taking a note of your overall mileage before fiddling with the Lidl computer as I managed to completely reset mine fairly easily (although I had a total mileage record on Cyclogs, fortunately).