First, you need to confirm that your computer supports 64 bit. What processor does it have?
After that, if it is a 64 bit processor, you will want to back up all your data to a thumb drive, online storage, extra hard drive, whatever you have access to. Switching over to 64 bit will require a complete wipe of your hard drive and reinstall of Windows.
Once you have everything backed up, download the ISO from Microsoft (or if you have the DVD for 64 bit, use that).
BEFORE you start the installation, make sure you have all the drivers you need, but most importantly, the driver for whichever network interface you use (wifi or ethernet). You can get those drivers from the computer's manufacturer website. Toss them onto the USB thumb drive.
Now install! Add drivers and start patching. Then put back all your data. You're done!