Only the 27" model has the access panel, the 21" model doesn't. I'm not actually sure if the RAM in the smaller one is soldered on or if it even has additional

Only the 27" model has the access panel, the 21" model doesn't. I'm not actually sure if the RAM in the smaller one is soldered on or if it even has additional slots. But even if it does replacing it isn't something you can do easily.