Temp gauge faulty

[ADS_V6]

hi taking the dash out on the weekend as after having a new stat,temp sensor and coolant change.it has been found after doing the trip reset dash trick thanks to dan@sjm,that my temp gauge needle sits around 3 lines lower than it should,how do i get at it to repair it,or worst case if i change the clocks can i reset the mileage on the speedo or not

Any help as this is annoying me not seeing the needle in the middle it only sits on 3 bars

Thanks ADS