If you don’t have dual boot system on your PC or laptop, then GRUB don’t show menu on startup.
If you want to boot with different kernel then you need grub menu to select appropriate kernel to boot. To get GRUB menu press “shift” during startup. Otherwise if you don’t want to use menu then update grub to select kernel by default.
You need following changes,
1. Change /etc/default/grub file to select default entry.
2. Then update GRUB using update-grub command.