careful while changing the GRUB you can make the backup file first
Step 1. locate your grub file in root directory find in /boot
Step 2. As the root user, open up the /etc/default/grub file in your favorite text editor:
gedit /boot/grub/grub.cfg
Now, edit the "GRUB_TIMEOUT" line to set the time:
Step 3. In terminal, run the following command to generate settings from /boot/grub/grub.cfg file.
update-grub2
Step 4. Reboot your system.