Ferrite is the grayish-black metal portion of the antenna. It should be fairly easy to loosen it and slide it out. If you do you are in for a hard time finding a replacement. Sometimes the coil is not wound on a cardboard form, and is wound directly on the rod. If so, you are out of luck.
Cost usually dictates that they use as small a ferrite rod or bar as possible.
There is usually plenty of room for a larger one inside the case. If not, or if there are components that interfere, don't try to replace it. Consider an add-on amplifier . If you are like me, you usually save parts from scrapped radios. I have a couple of dozen ferrite bars and rods.