High resolution camera
To remove the old lens from the high res bullet camera, use a pair of long nosed pliers to unscrew the lens from the camera body.
You can then simply screw in the new lens in the same way.
It is important to note that tightening the lens focuses the camera image, so when the new lens is fitted, it does not need to be fully tightened - to locate the best focus point, either record some test data and check the recorded video, or use a preview monitor to look at the live view.
Low resolution camera
Unscrewing the lens cap should reveal a small flathead screw in the side of the lens casing.
Use a small screwdriver to fully loosen the screw, then simply turn the lens around while looking at the preview monitor until the camera is correctly focused. If you don't have a preview monitor, make small adjustments and record some test video samples.
Note that the lens should not be in focus when fully tightened - when you locate the correct focus point then tighten the screw and refit the lens cap.