If you Dell laptop computer screen isn't displaying a full-screen image after you adjust its overall resolution, this means that you've changed to a resolution that your monitor doesn't support. You've either picked a resolution that is too large or too small for your laptop's built-in screen to handle. You can correct this by switching it back to its recommended resolution, using native Windows tools.
Right-click on your Dell laptop desktop.
Select "Screen Resolution.
Choose the "Resolution" drop-down menu.
Slide the vertical bar to the resolution with the word "Recommended" listed next to it.
Select "Apply," then click "OK."
If still issue exits then check Dell U2515H Monitor Manual- guideusermanual.com/product-name-dell-u2515h-monitor-manual&po=152606&lang=English