Start with the thermostat
Confirm cooling mode, a sensible setpoint and that the display is powered. If batteries are user-replaceable, follow the thermostat instructions. Avoid changing advanced installer settings.
Look at airflow
Check whether air is moving from the vents and whether the return filter is visibly loaded. A badly restricted filter can contribute to poor cooling or coil icing, but it is not the only possible cause.
Know when to stop
Do not repeatedly reset a breaker, open electrical panels or add refrigerant. Burning odor, loud electrical buzzing, repeated breaker trips or heavy ice are reasons to turn the system off and call.