DIAGNOSIS You've hit the main limitation of this model—it only has one HDMI port and one DisplayPort. To switch between two HDMI-only sources (like two laptops or a PC and a game console), you'll need some extra hardware, as the monitor itself can't switch between two inputs on the same port type.
FIX The simplest fix is to buy a small, passive HDMI switch. It's a little box with multiple HDMI inputs and one output. You plug both your computers into the box, plug the box's output into your monitor, and the box comes with a button or remote to switch between them. Look for an 'HDMI 2.0 switch' to ensure it supports the monitor's 75Hz refresh rate and resolution.
IF THAT DIDN'T WORK If one of your computers has a DisplayPort output, you're in luck. Buy a DisplayPort to HDMI cable (note: it's not the same as HDMI to DisplayPort). Plug that computer into the monitor's DisplayPort. Now you can use the monitor's 'Source' menu to switch between the HDMI and DisplayPort inputs without any extra hardware.
HEADS UP Avoid super cheap, no-name HDMI switches. They can cause signal dropouts, not support the full resolution, or fail to pass through audio if you're using the monitor's headphone jack. Spend a few extra dollars on a known brand.