DUELING OSes

UEFI system should be able to revert any unwanted changes caused by installing a new operating system, as long as you can get into the UEFI settings. The UEFI boots an image from the EFI partition or live media, and userspace tools (both on Windows and through efimgr on Linux) can change the boot order and, in some cases, break things. It’s generally expected, but not necessarily desired, that a newly installed OS will put itself at the top of the boot ordering. Often this results in Windows booting immediately and the only way to get to the UEFI settings to revert this is to hold the Shift key while shutting down from the start menu. An option to reboot to settings should appear.