XAP is a file format for Silverlight applications that is also used to install application software on Windows Phone 7 and 8 phones. With Windows Phone 8.1 Microsoft has replaced XAP by APPX to bring unity in app development of Windows Phone and Windows Store apps.

Anyone who wants to download a Windows Phone app that is not available in the Windows Phone Store can download a XAP file onto the SD card and install them manually.

If you have a link to a XAP file, here’s how you install it on your Windows Phone:

1. 1. Download the XAP file onto your computer.
2. 2. Connect your Windows Phone to your PC using a USB cable.
3. 3. Transfer the XAP file to your phone SD card. I suggest you create a separate folder calls Apps and transfer the file to that folder just to keep things more organized.
4. Note: You can also transfer the file to the Phone internal storage.
5. 4. Disconnect the phone from the PC.
6. 5. On your Windows Phone, tap the Store icon to open it.
7. 6. Tap the 3 horizontal dots icon at the bottom of the screen to bring up the menu.
8. 7. Tap Install local apps.
9. 8. Select the app or apps you want to install and then tap Install.

Note: You must have an active internet connection to install XAP apps as they need to be verified in the Windows Store.