If you bought the Retail edition, nothing. It should have came with the 64bit disks. If it didn't, you can try contacting MS for a copy. If you bought OEM (which includes if you bought it at Staples, Best Buy, etc) then you need to buy a new copy. Home premium is ~$110.
Just to make sure you understand...you can't "upgrade" a 32bit install to a 64bit install - you can only do a fresh install. You don't necessarily need to "buy" a new copy, though, as the COA key from the 32bit install you have will work with a 64bit install, too (the key is good for either one). You could...acquire (or borrow) a copy from somewhere/someone and use that to do a fresh install.