You need to remove the hex-head bolt in the middle of the shellplate. I use a small block of wood to hold the shellplate from rotating while the bolt is undone. Raise the ram a few centimeters while this is being done, to clear the ram off the indexing mechanism.

Partially removed bolt.

Sheelplate removed - shell retaining spring remains.

Shellplate removed.

New shellplate positioned, ready to be bolted down.

Shellplate retaining bolt inserted, finger tightened only at this point.

The shellplate retaining spring is what keeps the cases on the shellplate while the press is cycled.

It is a very convenient system, because a shell can be taken off and re-inserted at any stage, without tools.

Hex driver and a piece of wood to prevent the shellplate from rotating during bolt tightening. Ram raised enough to disengage the index mechanism during tighitening.