Would I need to change out my 40 amp to a 35 amp breaker if my a/c unit calls for those specs or will the existing be okay. Oh yea 10G wire is in place now. I'm talking about a 3 ton heat pump split unit.

Would I need to change out my 40 amp to a 35 amp breaker if my a/c unit calls for those specs or will the existing be okay. Oh yea 10G wire is in place now. I'm talking about a 3 ton heat pump split unit.
It would be best to follow the manufacturer's specs. The reason is the 40 Amp breaker won't clear a fault (short) at 35 amps and might burn up something in the new unit voiding you warranty. As a side, is the 40 Amp breaker and associated wiring original to the house? The wire MUST be sized at a lower ampacity than the breaker to prevent fire. I don't have my code book here right now but I think the wire size should be number 6 or maybe even 4 AWG.

I have a 3.5ton electric heat pump made by Gibson. I would like to know what size wire and breaker to use. On the side of the unit it says min 35 amp max 45amp. I don't know if I should use the lower amp rating or the higher.

This will all be new work.