Same as @Yassine mentioned, it's not possible at all to build UWP Apps for Windows 7, in fact, to be more precise you can't even build Universal Windows Apps for non-Windows 10 Operating systems at all.
So here we are not just meaning that you can't build UWP Apps for Windows 7, but also can't build UWP apps for Windows 8 & Windows 8.1 which are more close to Windows 10.
But there are alternatives:
Since UWP is just a result of multiple Microsoft frameworks evolution, you may simply build the kind of app you want using the technology that precedes it.
For Windows 7, there is the well known WPF technology.
For Windows 8, There is the less known Metro Technology (not advised to use it though).
For Windows 8.1, Microsoft Invented WinRT (Windows Runtime), which replaces the system core Win32 API. It can be used to build advanced core targeting Apps (it can also be considered a successor of Metro).
For Windows 10, Microsoft has finally released Universal Windows Platform Apps (UWP), which is an evolved version of WinRT, with far more reach and can be used to target the Windows OS family itself (Windows 10) instead of the device itself, which makes it easy to build cross-device Apps.
Basically summing up the explanation above, you can simply pick a technology of your choice to build your app. All frameworks above aren't backward compatible except WPF. which makes it the perfect choice if you wish to build a Desktop App. While if you are targeting Windows Store you'll have to go through the limited UWP.