I have it on 69 and its up to 82 - Thermostats White Rodgers 1F80-261 thermostat is obsolete, CONSIDER 1F80-361

If the thermostat has batteries, then replace them. If not then there's a problem with the Air conditioner. If you have a voltmeter, set it to AC then connect it across the red wire and the wire marked C. If you don't get 24 or more volts, then the power transformer inside the air conditioner has failed.