I tried this today as well and from my perspective you need very steady hand to swap the keys and not break any of the really fine connectors. After I was afraid that I broke something (I removed the whole white connector parts, which are still on the laptop on the picture). So I would not really recommend to remove keys if you don’t really know what to do.
If you still want to try:
It is like @soundboard6 said. The scissor connectors are on the bottom of the keys (the side which is nearest to your body - I have a Librem 15rev3 as well) so when you follow the description of @kate you should lift the key on the lower side (until the scissor connector is “disconnected”). After that you have to slide the key upwards (in the direction of the display) to release the key completely.
The key I removed and reassembled is arranged like the first one in this video: https://www.youtube.com/watch?v=oaFg60ZPpPs (00:00 until 01:11). In the video the call the scissor connectors -> narrow clips.
To put in back in begin with the upper side, slide it down (in the direction of your body) and connect the scissor connectors/narrow clips by just pressing the key down.