Alas, there doesn't seem to currently exist a way how to convert DASH (Dynamic Adaptive Streaming over HTTP) files to the original, or any other video format. Perhaps you will be able to playback it using media player, however without audio track which is not included in DASH file. DASH files are meant for watching only (like on Youtube) and not meant for downloading. Try to download the video in other format.
Open DASH file Open MPEG4 file