1.Phone must be rooted.
2.TWRP recovery must be installed.
Disclaimer: Please do it at your own risk, I am no way responsible if anything goes wrong or you ruin /brick your phone.
3.Reboot to recovery by switching off your phone and pressing volume up and power button simultaneously till TWRP menu appears.
4.Wipe → Factory reset → swipe right to factory reset.
5.Back→ Advanced wipe →tick Dalvik Cache, system and Cache → swipe to wipe.
6.Back→ previous page → previous page →Install → Select storage(stogae on which you have saved downloaded ROM)
7.Browse and select 360OSbeta.zip → swipe to install
8.Wipe caches → swipe to wipe