There are different designs of plate holders... if you can take a close-up photo of one and post it here, we might be able to offer more specific advice.
In the meantime, I've had a couple of different designs on my car so far...
• the first design had a couple of small holes visible on the face of the right and left hand edges. These were designed to take some kind of tool/key to 'unlock' the mechanism that held the holder closed. I never had the tool/key, but found that I could undo the holder by using a small flat-bladed screwdriver (push in, twist a little, and prise open/apart). A little force, and it 'clicked' open. The top edge then folds up, and the bottom edge folds down, releasing the plate.
• the current design I have, has no visible holes. The clips that hold it closed are hidden behind/beneath the outer edge of the holder. To open this one, it's a case of feeling for the area where the clips is holding it closed, and prising/pressing/pulling to pop the clip open. It then folds open (either up or down) along the longer edge.
More difficult to describe than it is to actually do... good luck, and I hope this helps!