Slide that metal tube off of the valve and you'll see a little copper clip sticking out of the top of the brass cartridge housing. TURN THE WATER OFF. The clip will pull straight up and out (you may need to grab it with needle nose pliers or a small pointed object to get it started). Now you're ready to pull out the cartridge. Follow the instructions included with the cartridge puller or with the new cartridge.
This is awkward, but...
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