You can't.
The 64 bit dvd is different.
But, if you have a 32 bit dvd, the activation code is still good on 64 bit.
Borrow or download a 64 bit dvd and you should be able to use it.
A windows 7 upgrade kit will cost the same as oem and include both 32 and 64 bit dvd's.
Upgrade from vista is possible. But, a clean install is preferred.