Hi. I had this issue also with 2 of our latest HP Elite x2 1021 G1 Tablets. I solved it by installing HP's SP78451.exe Update Drivers for Intel Skycam Damera Drivers for Windows OS right from the HP website. after I installed and rebooted, the Camera App then had the Rear Camera option to switch at the Top left corner.
Hope that can help someone else..
Cheers
Computerlady!