DIAGNOSIS This is a defrost system failure. The freezer has a small heater that periodically melts frost off the coils. If that heater fails, or the defrost thermostat that controls it fails, frost builds up non-stop until it becomes a solid block of ice.
FIX You'll need to manually defrost it. 1) Unplug the fridge and remove all food. 2) Leave the freezer door open. Place towels in the bottom and in front of the fridge to catch water. 3) Let it sit for 12-24 hours until all ice melts naturally. Do NOT chip or scrape the ice, as you can puncture the cooling coils. 4) Once completely dry, plug it back in. If the ice returns within a week or two, the defrost heater or its timer is defective.
IF THAT DIDN'T WORK If rapid ice buildup returns after a proper manual defrost, the internal defrost components need replacement. On this model, that's a significant repair that involves removing the interior freezer liner to access the heater and thermostat.
HEADS UP While it's defrosting, check that small drain hole at the back of the fridge interior (under the bottom shelf). The meltwater from this big defrost will need to go somewhere, and you want that drain clear to avoid a flood inside the fridge section.