iMac's do not have upgradeable processors. There have been some who OVERCLOCKED them, this is not recommended to the average user. Also concieveable would be a logic board swap, but the difficulty in this can be high, as well as cost. Your best bet is just to sell it, and put that towards a newer faster machine.
Apples are great, well designed, beautifully functional peices of computer hardware. However, they do lack in the upgradeability area. This is just one of the tradeoffs you make.