If you want to change the boot order on a virtual machine in vSphere, it’s not a setting in the vSphere interface. You have to force it to bring up the BIOS menu, and then change the order in the BIOS once you’ve booted up the VM.

Go to Edit Settings > Options > Advanced > Boot Options > Force BIOS Setup and check The next time the virtual machine boots, force entry into the BIOS setup screen

That setting will automatically revert to being unchecked once you’ve booted up the machine once with it on.