To remove most standard BlackVue cameras from the mount, you will need to firstly unplug the cable(s) on the side of the camera, then follow the image below - pressing and holding the 'Lock' button on the mount while simultaneously sliding the camera out of the mount:
 

Rear camera's do not have a locking mechanism and are friction held in place. To remove the rear camera, unplug the rear camera cable, then hold the mount firmly to prevent accidently removing this from the windscreen, then twist and pull the camera from the mount.