the file is located here - /etc/default/grub you need to be in root and open it in your text editor to make the change - below is mine from Expirion Linux which is based on Debian/Devuan
GRUB_DEFAULT=0
GRUB_TIMEOUT=3
GRUB_DISTRIBUTOR="Expirion"
GRUB_CMDLINE_LINUX_DEFAULT="quiet"
GRUB_CMDLINE_LINUX=""
where it says GRUB_TIMEOUT=3 in your case 5 just change it to what you want - or you can add a # in front of it - so it looks like #GRUB_TIMEOUT=3 this will skip the TIMEOUT completely after the change save the file then run from the terminal
If you find you want the TIMEOUT back just remove the # and run sudo update-grub again
Reboot machine and the delay will be gone