P1914S, where is the "aspect ratio" setting?

Hi!

I recently purchased a Dell P1914S to use alongside my Xbox One. Unfortunately, the aspect ratio is off, and my picture is compressed horizontally. I have tried to find the relevant setting on my monitor to adjust and correct this, however I can not find this. Does anyone have any possible solutions to this, or know where the setting may be? Any help is greatly appreciated!

Thanks!