If you want to change it, you would need to remove the top cover -
Take out the four screws (one in each corner) and remove the top cover. Then dig out grease with a screwdriver or possibly wash it with some degreaser. Actually, stirring it up may soften it enough so you don't need to get the old grease out. In any case you then squirt it full of new grease and put the cover back on it.
If you just want to fill it, you can remove the round plug by gentley prying it off. I did that on Edgar Kaiser's Henry J this spring. I just removed the plug. Look how dry it was inside!
I then took my grease gun and pumped it full, and popped that cap back on it. It actually shifted OK before but it's much better now!
Let me know if this helps.