When the computer turns on, it shows a screen with the following text.
At the top it says: GNU GRUB version 2.02 ...then in the box it says: * Trisquel GNU/Linux Advanced options for Triquel GNU/Linux ...then at the bottom it says: Use the and keys to select which entry is highlighted.Press enter to boot the selected OS, 'e' to edit the commands before booting of 'c' to a command-line.
The highlighted entry will be executed automatically in 21s.