The grub-menu does give access to additional options during boot, which is sometimes needed to bypass common issues.
Many system will not show a grub-menu on boot, but a graphical splash-screen instead. To show/enter the grub-menu requires you to use the "right" hotkey during boot in this case.
There are two common hotkeys to be used (where it just depends on your concrete setup, which one is to be used):
To enter grub-menu during boot press+HOLD the left SHIFT-key (in very rare cases users had to use the right SHIFT-key instead).
To enter grub-menu press (probably multiple-times/ but don't hold) the ESC-key shortly after keyboard-init (most keyboards shortly flash LEDs after init).
Some systems are not able to detect keypress-events when starting form a 'cold'-boot = fresh power-on.
It often does help to perform a warm-boot with many system = you init a reboot of the machine without turning it off before.
This way the hardware (keyboard) of your machine is already initialized and is expected to process keypress events somewhat more reliable.
Use the quit->restart offered via the "power"-icon (top right).
... with a blinking cursor top-left: Press CTRL+ALT+DEL to reboot
If you got a live-mint (the installer media), you can gain access to the grub-menu by using the chain-loading from there:
This should show the grub-menu of the installed mint, where you can perform your tasks as needed.
With a fresh install of linuxmint 19.3 one might not be able to enter the grub-menu using the typical hotkeys even after a warm-boot.
You can fix this by adjusting grub.cfg of the installed mint from a live-mint (the install-media) to show the grub-menu without any additional user action.
Instead of doing the required modifications manually, you can use this patch for mint 19.3: IRC:patch-grub19.htm
|IRC:patch-grub19.htm||grub-menu patch for mint 19.3|
|IRC:bootparam.htm||edit kernel params on boot, common use-cases|