That is actually not a center cap, but the Axel itself. Take out all the bolts and slide out the whole Axel. Then there is a Nut on the axel tube. Take out the nut "lock" and nut then take off the drum, although the drums sometimes can be stubborn. You may have to back off the brakes from the adjuster hole on the back side.
Now, and this is important, when putting the Drum back on you need to tighten the Nut properly, as there is inner and outer Bearings on there. Also make sure that the nut "lock" is in good condtion and properly installed. I would either refer to a repair manual or have someone very qualified to re-install it for you, although someone may want to jump in on here and explain it in detail for you.
It is also a good time to inspect the Bearings and replace the Seals and Gaskets. The whole job is really not a piece of cake and is not for the non-experienced.
Bill
