You must first remove the brew screen in order to replace the gasket, as it helps hold it in place. Remove the phillips head screw in the middle of the screen, then the screen itself. Make sure the inside surfaces of the brew head are clean of coffee or pieces of the old gasket, as these can prevent the new gasket from sitting flush. You may need to use a flat-head screwdriver to pry the gasket into place, but once you have it most of the way in you can attach your portafilter to help press it the rest of the way home. Then simply replace the brew screen and screw, and you're done.