The following forum link answers that question.
Welcome to the Dell Community @Ross204
You have a few options.
Intel Core i7-2630QM CPU Upgrade:
The i7-2760QM or the i7-2820QM look promising.
Make sure to update your BIOS before doing any Memory or CPU upgrade.
ALIENWARE M17XR3 A12 BIOS:
Alienware M17X R3 Service Manual (English Only)
Removing/Replacing the Processor Module starts on page 45