JAMAS, here are a few pictures for you.

This one is tough to see, but it's the outside of the roof rail. Notice the notches at the front and back, that's where the tabs on the rail covers snap in.

This is the back side of the roof rail. Again, notice the slots for the cover tabs.

Back side of all four rail covers. All of the tabs are perpendicular to the surface of the covers.

Cover Close-up.
I think, like others suggested, you have to wedge a screwdriver, or your fingers, under the bottom outside edge of the rail cover and gently pull away from the rail so that the tabs on the covers clear the slots in the rail. Then you can rotate the cover up until it comes off.
Hope this helps.
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