There is no upgrade path from 32-bit to 64-bit, it will have to be a clean install; after you have made backups of anything you want to save to external media from the Hard Disk Drive, the best way to proceede is to do a wipe (secure erase) of the entire HDD; have a look at these tutorials at the links below and be sure to post back with any further questions you may have and to keep us informed.
At the risk of stating the obvious - don't forget to deactivate your paid for 3rd party applications. They will need to be reinstalled and reactivated.
My Computer
Computer type
PC/Desktop
Computer Manufacturer/Model Number
Own build
OS
Windows 7x64 Home Premium SP1
CPU
Intel i7 2600k
Motherboard
ASUS P8Z68 Deluxe
Memory
G.Skill Ripjaws (DDR3-1600) 2x4GB
Graphics Card(s)
Nvidia GeForce GTS 450; Intel HD Graphics 3000(GT2+)