How to Access the Advanced Startup Menu in Ubuntu
If you’ve ever dual-booted Ubuntu with another operating system, you’ll be well acquainted with the text-based GRUB boot menu. The GRUB menu can be used to boot a different operating system, load an old kernel version, or boot into recovery mode. On older Ubuntu installations, the GRUB menu can even be used to launch Memtest, which used to come bundled with Ubuntu. However, on some systems, the GRUB menu is not shown by default during startup, and must be accessed via a keyboard shortcut.
Legacy BIOS Method
On systems that boot in legacy BIOS mode, hold the Shift key during boot to enter the GRUB menu.
On modern systems that boot in UEFI mode, press the Escape key during startup before Ubuntu loads.