Each version of Mendix is a separate Modeler, so you don't update the Desktop Modeler, but you can update your Mendix project.  If you have downloaded and installed 7.18.0 (found here:https://appstore.home.mendix.com/link/modelers/) then you just need to open up the 7.18 desktop modeler and then open up your Mendix Project.  The modeler will present you with a message asking you if you would like to convert your project in place.  It's a simple as that.