There are a number of ways to download the source code. Some are described here.
Option 1: The more complicated and easier to update method is using git. You'll need to download git or some client from here. When that's installed, right click in any folder and click on "Git Bash". When that opens, type in:
git clone https://github.com/Citadel-Station-13/Citadel-Station-13-RP.git
(hint: hold down ctrl and press insert to paste into git bash) This will take a while to download, but it provides an easier method for updating.
Option 2: Download the source code as a zip by clicking the ZIP button in the code tab of https://github.com/Citadel-Station-13/Citadel-Station-13-RP (note: this will use a lot of bandwidth if you wish to update and is a lot of hassle if you want to make any changes at all, so it's not recommended.)