You have to go into "Setup". There is an "Instrument Layout" page in the Engine section. At the bottom of this page you will see the timers. You can reset these as required.
Note there are 3 different timers available:
The engine timer increments when RPM is above ~500, similar to a Hobbs meter.
The flight timer increments when the GPS GS (or IAS if available) is above 35 knots.
The tach timer increments when the RPM is above a specified value, default is 2000 RPM