Change my complete system to 16 to 9?

As you might have guessed in the title, I would like to change my complete system to 16 to 9 (so that, for example, games like Fortnite can be played in 16 to 9 format). I had bought a 21 to 9 Ultrawide monitor a few weeks ago and would now like to use the monitor just in 16 to 9 format. My PC consists of an AMD Ryzen 5 and a GTX 1060.

(And yes, I realize that I'll have black borders on both faces)

Would appreciate your help!

Presumably you need "only" the monitor to 16: 9 festeinstellen.