The best method to unclog this drain tube is with hot water. Unplug the unit from the wall outlet before attempting this. Remove the back cover panel and you will have access to the evaporator and drain trough. Remove all the ice from the trough and now you will be able to pour the hot water slowly and directly on the drain hole. This can take awhile depending how deep down the icing took place. You can also try inserting a piece of wire into the drain hole to help clear it. If you do this be very careful not to puncture the evaporator coils or punch a hole in the drain tube or cabinet insulation. Hot water works best. If you need more assistance you can resubmit your question with more details.