Drag the left edge of a keyframe to change its duration. You can also click a keyframe and change the duration in the keyframe inspector side bar.
This tells Rotato how you want the mockup devices to move from keyframe to keyframe. Click a keyframe, then change the easing in the side bar.
Click a keyframe, then change the camera rotation as you would normally. Once you're done rotating, you'll note that the keyframe thumbnail updates, reflecting the new rotation.
Delete a keyframe
Select the keyframe, then click the trashcan. Alternatively, press the backspace button on your keyboard.
Delete all keyframes
De-select all keyframes. Then click the trashcan.
Duplicate a keyframe
Select a keyframe, then click the + button.