I did not cover my AC last year either and do not have problem with AC
now. If your AC is too old, it may make sense to replace one. Call
some companies for a price at first. If it is too expensive, get a new
one.
On the thermostat,set the fan to "auto", set the function to "cool", set the temperature to 23-25 when you are home and set to a higher temperature while you are away from home.