Freezer won't freeze, from other posts it seems it is a defrost problem. I remove cover from inside back of freezer and an ice ball had formed on the top od the coils. I defrosted it and put it back together and turned back on. Smae thing not freezing and ice ball formed on top of coils. How do I replace the defrost thermostat?

Yes it could be low freon

Here is a link, take a look at the 3 example pictures part way down the page.
http://www.applianceaid.com/frig_notcold.html

If it was a defrost thermostat problem then your coils would be iced/frosted over completely.
Also if it was a defrost problem once you manually defrosted it, it should have rum OK until the ice/frost built up again on the evaporator coils to the point where the fan could not pull air through them.

If it is low freon it will be an expensive repair as you need a pro and there is no guarantee that they will even be able to find the leak.

Often it makes more sense to invest those dollars into a new unit.

I did look up the manual and it looks like the sealed system has a 5 year warranty so you may want to check into this. Be sure to read the fine print.