I have a mobile home & when it was set up in 96 we had separate cooling & heating units run off one thermostat. We had a 3 ton ac unit & an electric furnace inside for heat. This past summer we bought a 4.5 ton ac & heating combined unit. We hooked up to wiring that old ac unit was hooked up to and a/c has worked fine through summer. Winter is coming & I need to change old thermostat to a new digital thermostat but how do I do it and get the combined unit's heat to work and not the inside electric furnace?

I assume the new unit is combined and all the equipment is located outside?

If so you will have to pull a new 5 wire thermostat cable outside or splice your original 5 wire cable to the new cable going outside. Wiring according to the instructions that came with your new outside unit and your new thermostat.

When you splice the wires make sure to maintain color to color ans that the splices and of good quality.

Now if your old system had only 2 wires going to the original thermostat with a change over switch on the old furnace then you will have to replace that 2 wire cable with a 5 wire cable.