Sometimes I forget that GRUB as limited time for you to select your OS and when the time comes, the PC boots from the default OS when I actually wanted another one. After doing a bit of research, here's how to disable or change the GRUB time for OS selection.

Simply open the terminal, and run this command:

sudo gedit /etc/default/grub

On "GRUB_TIMEOUT", change the value (in seconds) according to your needs (0, for instant boot to the default OS, -1 for unlimited time).

For example, mine is:

GRUB_TIMEOUT=-1

Since I want to manually select the OS.

Save and close.

Now run:

sudo update-grub

And that's it.