Assuming this is a laptop or just a second display, what you have right now is called extended desktop as opposed to display mirroring which you appear to be looking for. You can change this in the same place you change the display resolution. Just be forewarned that you can't mirror two displays with different resolutions, so if the Dell monitor is, for example 1080p and your laptop's built in display is something like 1367x736, the Dell monitor will take on the laptop's resolution.