Hi.
If all is working OK - heating switching on and off as it should, and it is just a display fault, there is nothing you can do other than replacement.
If the screen is totally blank, and is not controling the heating, there could be a wiring fault.
this would depend on the boiler you have.
To replace the timer, power off. Two screws underneath, and unplug from the backplate. As with other Drayton controls, no need to replace the backplate.