Is there a decorative cover on the cap? Often, you can pry that off to expose a screw and then remove the cap. A picture would help. On one of them, the cap has a decorative cover that snaps onto the base portion; you pry that off to expose the screw to then take the whole thing apart. This is one reason why it is very useful to keep the installation instructions, as they show the parts breakdown and often make it easy to figure out how to do maintenace later one. If you can identify it on their webpage, they usually have the installation instructions/parts breakdown there to help you.