If you're already using Windows 8, you can download and install it right from the app store. People who use Windows 7 and earlier need to download the ISO files from Microsoft's page.
Either way, make sure you back up your computer! Though Windows 8.1 has been tested thoroughly, it could do some weird things to your computer. It's better to be safe than sorry.
There are some hardware requirements for Windows 8.1, as well. Your computer needs to have a 1 gigahertz processor, 2 gigabytes of RAM and 20 GB of hard drive space. These are the minimum requirements, so more is better.
You also need to know how to make a bootable disc. This download will help you do it, but it still requires a bit of knowledge.
Whatever you do, do not install Windows 8.1 over your current version of Windows. This will nuke all of your programs and settings. If you didn't back them up, you'll be out of luck.