You cannot change to 64 bits unless you do a clean install by using a 64 bit disk. Your processor must be capable of running 64 bits.  windows.microsoft.com/.../32-bit-and-64-bit-windows Here's some advice--It is not worth the expense of about $100 to buy 64 bits win 7 for a model that is a few years old. Instead, wait until the summer or early fall when windows 10 will be a free upgrade for windows 7 users. You must be running windows 7 with the latest update to qualify for the free upgrade. 
I hve a Dell Inspiron 1545 hich has Windows 7 32 bit on it. I would like to change this to Windows 7 64 bit. I don't have a Windows CD, it must have been loaded when it arrived.  How can I make this change easily and will it cost me or   Thanks  Whit
Looking for an easy way to migrate from Windows 7 (32 bit) to Windows 7 (64Bit)