i don't think you can. it upgrades based on what you have and not what you want to have.
you can talk to MS and see if they will swap your key to a 64-bit one but other than that, there is no way to just change it from 32 to 64-bit.
From what i have gathered on the web It IS possible to upgrade to windows 10 64 bit from windows 7 32 bit by upgrading to windows 10 32 bit and after that upgrading to 64 bit, it's just that i don't know the steps between upgrading from windows 10 32 bit to 64 bit.
i don't know either since i have never heard of it being done nor done it myself. good luck, all i can say is to google until you find it. never seen it myself so can't offer any ideas of where to go.