To get the boot menu when 0 seconds is enabled, press ESC when the menu flashes.

update-grub

update-grub is a stub for running grub-mkconfig -o /boot/grub/grub.cfg to generate a grub2 config file.

Boot Windows 10 from grub

insmod part_gpt
insmod chain
set root=(hd0,gpt1)
chainloader /EFI/Microsoft/Boot/bootmgfw.efi
boot