I want to change windows 7 32 bit to windows7 64 bit how can i do it

You need to do a clean reinstall.

If you can borrow a 64 bit windows 7 dvd, you can use it to install the 64 bit version, and still use your current product code to activate.
Microsoft used to send you a 64 bit version of vista for a handling charge of $10 if you wanted to upgrade from a 32 bit version. Don't know if that is available for windows 7.