Welcome to the forums,
look instead of messing with your boot [which can cause problems] here is a fun thing with most Linux distributions,
1] open your software manage, In the search box type cMatrix, and enter
2] provided your distribution includes it, you should have a download screen, select install and let it run]]
3]from your desktop, open the terminal , expand to full screen, type into the terminal cmatrix and enter.
4] to close, just shut down the terminal window
And take Note of what
@bob466 said, and do not trust any YouTube video's, before you do, check written documentation, many video's are lacking in the details needed to make safe changes to your machine