Also check/clean the filter and check for any air obstructions, or if the fan is dying. The air passing over will more or less warm up the coils enough so they don't freeze, but like an AC unit, if there's not enough air, it will freeze.
If it is indeed low on freon, there is probably a leak. You could maybe get an hvac person to look at it, but unless it will only cost like 30 bucks or it's a big expensive unit, probably not worth paying too much in repairs. If you dispose of it, treat it like a fridge or AC unit, you can't just bring it to the dump.