What do the pilots do if the plane they are operating does not have an anti-ice system? Anti-icing equipment is designed to prevent the formation of ice, while deicing equipment is designed to remove ice once it has formed. These systems protect the leading edge of wing and tail surfaces, pitot and static port openings, fuel tank vents, stall warning devices, windshields, and propeller blades.

.What if the plane does not not have any anti - ice systems
Here is my research about one type of plane that does not have an anti - ice system and what do the pilots do to deice the plane.

A Cessna 172 has no anti icing system, as long as I remember a electrical pitot heater for preventing freezing over the pitot tube and a carb heater for preventing formation of ice in carburetor are the only two ways to prevent ice from forming; for that the C172 cannot fly in icy conditions.

I have researched about this topic and I am still looking forward to learn from the users who answer my question.
If an aircraft is not equipped with anti-ice or de-icing systems, than the answer is simple: You don't fly into icing conditions. This is usually listed as an operating limitation in the POH/AFM/FCOM of the respective aircraft. For a Cessna 172, you can e.g. find:

KINDS OF OPERATIONS LIMITS
The Cessna 172S Nav III airplane is approved for day and night, VFR
and IFR operations. Flight into known icing conditions is prohibited.

(C172 POH, Section 2 Operating Limitations)
If you are approaching icing conditions, there are usually two options to avoid flying into icing conditions:

Reduce the altitude: The temperature is higher at lower altitudes. If you can reduce your altitude enough to raise the temperature above 10°C, you are out of icing conditions.

Change course: If you see a cloud ahead and the temperature is below 10°C, you cannot enter the cloud because that would mean you enter icing conditions. You can however deviate from your current course and fly around the cloud.