DIAGNOSIS Excessive frost in normal temperatures usually points to low airflow across the evaporator coil. That coil needs a steady stream of warm, humid room air to absorb moisture. If that airflow is restricted, the coil gets too cold and frosts over.
FIX The number one culprit is a dirty air filter. Remove and clean the foam filter thoroughly, let it dry completely, and reinstall. Next, with the unit unplugged, carefully vacuum the coil fins through the rear grille and the front outlet. They can get packed with dust and pet hair, which acts as insulation.
IF THAT DIDN'T WORK If airflow is good, the issue could be the refrigerant charge is low (causing the coil to get colder than designed) or the defrost sensor/timer is faulty. The unit should periodically cycle the compressor off to let the fan melt any frost. If it doesn't, frost builds up. This is a sealed system or control board issue needing a pro.
HEADS UP When you see heavy frost, turn the unit off and let it completely thaw before restarting. Running it with a fully blocked, icy coil can burn out the fan motor or cause liquid refrigerant to flood back to the compressor, which will kill it.