Founded it in the FCPX Help
Switch angles in the Inspector
Switching angles in the Inspector allows you to switch angles for multiple selected multicam clips at once.
Select one or more multicam clips that you want to switch.
Open the Info inspector.
In the Info inspector, do one of the following:
To switch the video angle: In the Active Video Angle pop-up menu, choose the angle you want to switch to.
To switch the audio angle: In the Active Audio Angle pop-up menu, choose the angle you want to switch to.