This wiki shows how to replace the nozzle.
Before removing the silicone cover, make sure the nozzle is not reheated and the temperature is below 40 degrees
Reheat nozzle to 230 degrees Celsius
Please wear gloves when removing the nozzle. After the nozzle is removed, a small copper pipe will fall out of the nozzle. Wearing gloves can prevent burns.
After removing the nozzle, the yellow part in the picture below will fall off, please be careful of getting burned.
After removing the nozzle, use a steel needle to extrude the material
If not, replace the hotend and meltzone set (step 11).
Install a new nozzle and conduct an extrusion test.