Generally, the cooling loops in a freezer are continuous, so if the bottom is working then the top should be cooling too, but I suspose it is possible some have separate coolling loops in the unit. Run your hand around the inside surface of the freezer to see if the upper inner surface is signficantly warmer than the rest. IF the upper coils are warm, check to be sure the fan and evaporator coils (under or behind unit) are clean - dirty fan or coils reduce the cooling efficiency and cause the coolant to run hot, which can result in warm coils at one end of the cooling loop in the reefer walls.
My guess - you have a torn door seal or seal leak or a misaligned door hinge that is letting the cold air escape, and of course heat rises so the warmer air would be at the top. Put a strong flashlight inside the freezer facing forward and close the door, then with room lights off (at night) check all around the door seal for light leakage. You can also take a 2" strip of ordinary paper and close the door on it, working all around the way around the door seal - checking at each location to see if you can pull the strip out without significant resistance.
Some seals are pop-in or easily changed, some require removing a screwed-on plate, some are glued - google for the maintenance manual and parts list for your model if inclined to DIY. Otherwise, Appliance Repair - Large is your Search the List category. If you are sure it is the door seal, having them order before coming will reduce the trip charges.
If hinge, read in owner's manual how to adjust - usually just loosen a couple of bolts on the end of the door that is maladjusted, press the door in firmly so the seal is tight, then retighten bolts. Do NOT totally remove the bolts - can be a pain to reengage with the threaded plate in the reefer at times - just a half turn or so should do it.
