If you change your physical connections into the ATEM Television Studio, this will move where a particular camera shows up in the Multiview. So for example, if you have two HDMI cameras and Camera A is plugged into HDMI 1 and Camera B is plugged into HDMI 2, Camera A will be show up in the Camera 1 spot on the Multiview, and Camera B will show up in the Camera 2 slot. However, if you take Camera B and place that in the HDMI 1 Slot and take Camera A and place that in the HDMI 2 slot, Camera B will show up in the Multiview's Camera 1 and Camera A will then show up in the Camera 2 spot.
Do note that in terms of the Multiview, HDMI 1 will always correspond to Camera 1 and HDMI 2 will always correspond to Camera 2. In the same vein, SDI 5 will always correspond to Camera 5 and SDI 6 will always correspond to Camera 6. Inputs 3 and 4 are the only switchable ports in terms of HDMI versus SDI. You can change the input type for these ports in the software control panel. However, whichever you select as the input type, the port labeled Input 3 will always correspond to Camera 3 and the port labeled Input 4 will always correspond to Camera 4.
I hope this clarifies rather than confuses. If I've completely missed the mark on the question regarding physical connections, please let me know.
*Edited to add: The post from Thomas right above mine might explain things a bit more succinctly