Freezer is not defrosting properly , I already change the thermostat, my refigerator model is TBX74UL01 OR TBX74UB01.

If the thermostat is replaced, but still its not defrosting, then two possibilities, either weak defrost heater,Heater is the part which heats and it defrosts, if the heater gets weak, then unit will not defrost properly.And it will take more time to defrost, and till that time, again frost gets collected.---------- But if defrost heater is ok, then problem is defrost board.The defrost board is the part, which controls both thermostat and defrost heater.-----------
This problem is called defrost problem.-------------

So first thing you try is , perform, manual defrost reset procedure.---------

The manual defrost reset procedure is as follows : ----
First unplug the unit from main power outlet. Then empty whole unit completely. Then keep both the doors open. Wait for 24 hours. All the ice collected on the coils and in the freezer section will get melted. Clean the water from the sections. Then Connect the power cord of the unit to main power outlet after 24 hours/.Then set the temperature to normal in both fridge and freezer section. Then check,if same frost is getting collected, then its defrost problem,. If it's again building up ice that means the unit is not defrosting. It's starting the cooling procedure, and then while defrosting, it's not working. The defrost assembly consists of three main parts. The defrost thermostat, the defrost heater and defrost board. Any of the one is not functioning. When thermostat sense the maximum cooling, it triggers the defrost heater to start heating and this defrosts the ice collected on the coils and the system works. Both thermostat and heater are controlled by defrost board. First the thermostat and defrost heater to be checked, if both check ok by meter and visually, then its defrost board causing the problem. IN YOUR case, defrost thermostat is already got replaced, so ignore thermostat, but other two possibilities needs to be checked.----------You can get required parts from from online sites like: --- www.repairclinic.com or from searspartsdirect online site. --------- This should help.Thanks.Helpmech.