There is a smaller nut, visible in the photo, which secures a stem packing sleeve. The larger hex you see will unscrew the entire stem for repair or replacement. FIRST, of course, turn off the water, and OPEN the stem. Don't try to unscrew it while it is dogged down in the off position.
I don't see any setscrews under the flange, so the sleeve is probably screwed onto something. Sayco parts are available, so don't be overly concerned if you have to use some pliers on the parts, which will damage the finish. You can get new ones. Finally, from the hardware store, for about $12, you can get a set of shower sockets, which are just deep sockets in odd sizes used by faucet manufacurers, like 29/32", etc.
You can get replacements for all the gaskets and seals, inside, but a whole new stem won't cost that much more. Also, behind the stem is a removable brass seat. It is remvable with a square end or for sayco I think the hex end of a Shower Seat Tool, also available for a few bucks at the hardware store.
Some hardware store will have a "complete kit" for your sayco, which will include stems, seats, and all new chrome and handles. Might be worth it. Take your stem with you to the store, however. There are "new style" and "old style" sayco stems.