How to sideload app on Windows Phone 8.1 now?
For Windows Phone 8.1 app, you could deploy and test your app in Windows Phone Emulator first. While it would be highly recommended to test your app on actual device before you upload it to the Store.
For sideloading Windows Phone 8.1 app, you could try to use the Application Deployment tool:
• First you have generate your app package
• Then make sure the device is registered and connected
• Find the Application Deployment tool in Start menu
• Run the tool and browse the package file (.appx) then click deploy
For details, please refer to Deploy Windows Phone 8.1 apps with the Application Deployment tool.