Venting a finished attic
Spaces are inside or outside, never both at the same time!

What you need is not venting as this attic is conditioned space -- it's inside the house control envelope, in other words.  What you have is likely an issue with improper ductwork that isn't returning hot air from the attic efficiently -- the temperature buildup is caused by a combination of stack effects (hot air rises) and the inability to get this hot air back down to the air conditioner to be cooled.  If there are returns from the upper attic down to the HVAC system, check to make sure they are not closed off by a damper, and that the HVAC system is properly sized for the load as per Manual J.  (HVAC sizing is like Goldilocks -- not too big, and not too small, but just right for the house you have.)