@balajik I guess with Windows Device Recovery Tool you would get WP8.1 back on your Lumia 720. I guess your phone may have out of memory to install W10M. I never faced this issue on Lumia 830 which has 16 GB internal memory.
@AbhishekMSharma I guess you can try with Windows Insider to get Preview version on W10M. Latest .164 is quite stable, and it is also sent as Production version for official release.
@AbhishekMSharma I guess you can try with Windows Insider to get Preview version on W10M. Latest .164 is quite stable, and it is also sent as Production version for official release.