There are "Windows Phone 8.1" project.
I have only "Windows Phone 10" device.
App work on "Windows Phone 10 emulator" but not work on "Windows Phone 8.1 emulator"
I need test app on device with "Windows Phone 8.1"
I found remote people with "Windows Phone 8.1"
I create package in Visual studio
How install *.appxupload on another device (TeamViewer o Remote Admin available)
According to your description, you need to sideload your app. First you need to package your App and here is the link of how to create a sideload package.
You need to pay attention to this part and should choose No option button for sideload. If you choose Yes, an .appxupload package will be generated and you can submit your package to the store.However if you want to test locally,you need to choose No option.
After you created the package, You can deploy apps by using the Application Deployment tool and here is the link.
Best Regards