Alas, it seems that no way how users can convert DASH (Dynamic Adaptive Streaming over HTTP) files to the original or any other video format exists at the moment. At best, you will be able to playback it using media player, however without audio track which is not included in DASH file. Even though some programs advertise it, dash to flv conversion (export fo Flash) is highly unlikely.
Generally speaking, .dash files are meant for watching only (like on Youtube) and not meant for downloading. But you can always try to download the video in other format.Open DASH file Open FLV file