First thing you should check for is a proper freezer door seal all way around the door opening,especially along the bottom. Articles blocking the air vent tunnel forcing air to the bottom of the compartment.

Your defrost drain opening and tube are located behind the back wall of the freezer compartment. it is an internal comonent and cannot be replaced at this time.

You can attempt to check and clean the tube if it is frozen or clogged.
You would need to remove everything in the freezer compartment, remove the screws holding the metal wall to the liner and carefully pull forward at the top and disconnect the wiring harness,and remove the panel.

You should find an opening in floor of that compartment (approximately 1/4 inch diameter)this is your drain tube connection. If frozen deice using a hair blower/dryer. You wold then want to carefully pass a length of wire or smaller tubing down the opening until it is clear and / or proruding out the tube in the back compartment over the defrost drain pan.