Force Airport Express to join 5GHz network
Your computer will use the wifi best suited under current reception characteristics. If you want to force your windows computer to only use the 5GHz wifi, you'll have to remove the 2.4 GHz from the known networks as for example detailed here for Windows 10 (which you appear to be using judging by your screenshot): http://www.ilovefreesoftware.com/24/windows-10/remove-saved-wi-fi-networks-windows-10.html

What you could do is configure both wifi's with the same name and let your computer switch automatically (works way better when they have the same name), that way your laptop would choose the 5 GHz one when close enough to the Airport and choose the 2.4 GHz one when further away