That tutorial looks very out of date.
First upgrade to WAMPServer 3.1.1 available from here it is the secondary repository, but it is much easier to navigate than SourceForge. Of course you should be able to find all this on SourceForge if you prefer.
You must be using WAMPServer 3.0 or above to run this upgrade, and I assume you are, this will just update any 3.0.? version to 3.1.1 without changing any versions of Apache/MySQL/mariaDB/PHP or any of your code or databases.
Then go back to the linked page above and find the PHP-ADDON that you want to install. The latest versions will all show automatically, if you want a non latest version then press the "See All Available Versions" button and all the versions of PHP/Apache/MySQL/mariaDB will be displayed.
Click on the version of PHP you want and it will download an installer for that version. Save it somewhere on disk and then run it. Once the new version is installed you will then see WAMPServer terminate automatically.
When you restart it you will see the new version available for selection using the wampmanager menus, to activate the new version of PHP just do
wampmanager -> PHP -> Version -> version numbers
Click the version number of PHP that you want to activate.