Switch the air conditioner to fan-only mode for an hour or so until the ice starts to melt. Without the cooling coils switched on ice is free to melt from the coils, frame and grille. Switch the air conditioner off completely if the icing is very bad.
Turn down the air conditioner to a lower setting. Ice can build up on the cooling coils when the room temperature gets close to the temperature of the air leaving the air conditioner.
Check nothing is blocking the flow of air out of the grille. Blinds, sofas and armchairs can all restrict air leaving the grille and cause ice to buildup on the cooling coils. Turn the air conditioner down or off for a few hours to melt the ice.
Move the air conditioner to a larger space for better results. All air conditioners have a BTU rating that indicates the strength of its cooling capacity. Ice can build up on the cooling coils, front grille and casing if the BTU rating is too powerful for the size of your room.