My master bedroom is above our garage which gets very hot and has no insulation or ventilation. During the summer, the master bedroom is much warmer than the rest of the house. Looking for a solution. Should I have another duct installed or will venting the hot air out of garage via fan provide relief?

Insulation and ventilation possibly good start. Possible to Insulate between Garage and Bed?

Depending are where you are located you may need to install some type of air conditioning , maybe just a window unit may make a big diffrence. I would insulate for sure.