Boot up your PC and remember what Operating System you want to use. My PC at the time is currently defaulted on Ubuntu and I want to default it onto Windows.
Open up Ubuntu.
Find the terminal. To find the terminal on Ubuntu, hit the circle on the bottom left and the terminal app should be underneath the clock
Once there, put in the command "sudo cp /etc/default/grub /etc/default/grub.bak" it will require your password which is just your login password. It will not appear when typing it in but once you have typed it, hit enter and you should continue.
(Sudo is the SUPER COMMANDER whilst Cp is also the copy, so if you stuff up /etc/default/grub, using the Cp, it will save a copy of it making it into a /etc/default/grub.bak)
Once that's done, put in the command "sudo nano /etc/default/grub"
After you put that command in, a big batch of commands will be placed in front of your screen, don't get overwhelmed, you just have to use one thing with it.
You will want to go to "GRUB_DEFAULT=0"
Head back up the Grub settings, each Operating System has a number, the top on is "0" and the number goes up per system on your PC, use your arrow keys to move down to the "GRUB_DEFAULT", for myself, the windows OS is the 4th one for me, so I will change "GRUB_DEFAULT=0" to "GRUB_DEFAULT=4".
To exit, hit "Ctrl + X"
your PC will ask to save the modified buffer which you will just hit 'Y' confirming to save it.
It will take you back to the original command prompt after you typed down the Sudo Nano command and type down "Sudo update-grub"
It'll run a command by itself
After that, restart your PC, the grub should pop back up but should be locked onto the specific OS you want, either hit enter or let it run, and it should boot up for you. Remember how my grub was originally locked onto Ubuntu? Now it will be defaulted onto "Windows Boot Manager".
And you're done! If you still have some other issues, ask your teacher about it and they hopefully should help out with any other issues.