Access GRUB Boot Menu
Restart your PC from Windows 11 by holding the Shift key while clicking Restart (from Start menu > Power > Restart). This opens the Windows boot options screen. Select "Use a device" or "Troubleshoot" > "Advanced options" > "UEFI Firmware Settings" to enter BIOS/UEFI setup.​​

Change Boot Order in BIOS
In BIOS/UEFI (keys like F2, Del, or F12 during boot), navigate to the Boot tab. Set the Linux entry (often labeled Ubuntu, Linux Mint, or GRUB) or the Linux EFI partition as the first boot device. Disable Secure Boot if Linux isn't detected, save changes (F10), and exit to reboot directly into GRUB menu.​​

Select Linux from GRUB
On reboot, GRUB appears automatically (brief delay, 10 seconds default). Use arrow keys to highlight the Linux entry (e.g., "Ubuntu" or distro name) and press Enter to boot into Linux. Windows remains as another GRUB option for future switches.​​

Switch Back to Windows
From Linux, simply reboot or shutdown; GRUB reappears for OS selection. If GRUB skips to Windows, hold Shift during Linux boot or edit GRUB timeout via sudo nano /etc/default/grub (set GRUB_TIMEOUT=10), then sudo update-grub.​