You are downloading the WP10 Preview XAP file v188.8.131.52 for Windows Phone. WP10 Preview is a free and useful Productivity app: The Windows journey continues... Try this preview-app to discover all windows 10 news! (it's just a demo app . Your phone will still work ... You can now download the offline XAP file to install & try it.
Download Locations for WP10 Preview 184.108.40.206 XAP file
Important Notice: Now Due To Unspecified Limitations, You Can Not Use The Offline XAP File To Install It Any More.
To try to install it manually from an SD card, you should choose one of the server location above to get the offline WP10 Preview XAP file, move the file to your phone's SD card and then tap 'Install local apps' in the phone's App list.