It is pretty easy! Hold the heater block with a 3/4″ wrench (it will try to spin as you back out the nozzle, which we can’t allow), and then unscrew the nozzle with a 9mm socket. The nozzle has a Teflon type guide tube that may come out with it; if it stays in the heater block it will be OK to pull it down. If the tube is blocked solid or causing a constriction (it will swell with time and heat), replace it with one of the spares that came with the printer.
When you reinstall the nozzle, be sure to hold the heater block again, and you should find it locks in place when the nozzle is tightened.
Looking at the provided images, although hard to see, it appears there may be a misalignment in the extruder between the steel pulley and the exit notch. If this is so, it will put a bind on the filament and give the flex too much resistance to movement. You can remove the Phillips screw holding the plastic support plate, and by flipping the pulley, you can sometimes get better alignment between the two.