To boot a different kernel-version using grub-menu, select the Advanced options for ...-entry and press ENTER (aka RETURN).
The kernels are listed in descending order, with the newest kernel first (which is identically to the default-kernel being used).
With current versions of linux (mint) there are 2 entries per kernel version shown, where the first entry represents the regular boot options for the kernel and the 2nd entry is used for the recovery-mode.
As you are reading this tutorial, you are probably facing issues with the current default kernel and likely want to test the previous kernel version first, which is the 3rd entry listed .
If you are trying to sort out issues with your hardware, like not detecting sound- or wifi-/bluetooth-devices or having issues with acpi - you might want to test a lts-kernel (with linux mint 19.x a 4.15 kernel) instead.
After selecting a kernel you have press ENTER (aka RETURN) to continue booting it (available hotkeys are generally shown at the bottom of the grub menu).
The changes are not permanent = they will be gone on the next reboot.
Which is fine for most situations, as one does often just wants to check if using a different kernel version would help at all with some issue.
|IRC:bootgrub.htm||entering grub-menu on boot|