Can I loosen the long center muffler screw and turn the rear half of the muffler so it’s pointing down away from the wing? (Evolution .46NT)

Loosen the nut first. It is a locknut, then loosen the long screw, rotate the end, re tighten the screw, then retighten the locknut.

It looks similar to an OS muffler. If this is the case, then yes you can. Just loosen the nut on the rear of the muffler (looks like you have already), and unscrew the long screw on the front of the muffler. Once loosened, the muffler can be adjusted to any diredtion to best suit the aircraft. Once done, tighen the screw down, and tighten the nut, and you're done.

Not a bad idea to put some BLUE loctite on the nut when you tighten it back up.

These things have a habit of vibrating loose and falling off!

Slap a silicon deflector on the tip as well.... I point mine straight down... you'll basically only get some goo on the tail

tie a piece of braided dacron fishing line or other strong but thin string around the near end of the rubber exhaust deflector (if you use one) and tie the other end to the screw before you tighten the lock nut. you will never loose a deflector.