If you're not interested in RE6, just explain to your mom that you're really grateful and that it was a good gift, then ask her if it would be alright to exchange it for Halo 4, because Halo 4 might be a better investment (you'll get more out of Halo in the long run, multiplayer, yadayadayada).
As long as you're not a jerk about it, I would imagine she'd be fine with it. Just be polite and make sure she understands that you're really grateful for the gift, but Halo 4 came out and you'd like to exchange RE6 for Halo. It's still her gift, so if anything, you're making the best use of her money by making sure it goes toward the game you really want.
if you haven't opened the plastic yet, you MAY be able to trade it for Halo.
this is probably the part of me that hasn't yet given up on humanity talking though.
Yea i know I can just do that but I feel likes its rude to say yea thanks but I wanted halo I can trade it in because she did buy it for $60 so at gamestop unopened games can be traded in within a week but I don't want to be rude to my mom
I think your mom would want you to have a gift you truly want. She'll be more satisfied to know you're playing the heck out of Halo 4 (cause it rocks) than later on, notice you've barely touched RE6 (cause it sucks).
say thanks mom, i really appreciate the gesture, you're such a great parent. though, and it makes me feel bad to say, i've already played this game, and even though it means the world coming from you, there is another game i've really been wanting that i haven't played yet. i was wondering if you could exchange it for halo 4, because i still want it coming from you.