how to change digital central heating timer to non digital ?

I should think it can be done quite easily - but an experienced electrician would be a wise precaution.

Central Heating timers have mains power going in (as you would expect) and their 4 unique outputs are Hot Water Off - Hot Water On - Heating On - Heating Off.

How they get connected will be a matter of the system they connect to - the engineer will know.

This answer is mostly guesswork.

Good luck...

Peccavi
March 2013