This is a simple way to determine if the problem is an easy fix or not. Unplug the unit. Remove the back panel in freezer section. Be careful, edges and evaporator fins are sharp. If the frost/ice is only covering part of the evaporator coils you have an issue in the sealed system (restriction, leak, compressor).
If the frost/ice covers the coils evenly the problem is usually in the defrost circuit (Defrost heater,thermistor, defrost timer, evap motor).
Any seal system repair will be a major and costly repair requiring a certified technician. Hope this info helps!