Go to TWRP recovery slide right and select backup,now You can choose whatever to backup,I select all,select location SDcard,press backup, now system is also backed-up.now reboot device. Go to settings,advanced settings choose backup and backup your apps and settings. This Will not be erased afther clean restore. Download the rom You want, I have file Explorer app from Playstore with root.I Place the download zip on my SDCard.Go to TWRP again,go to wipe and advanced wipe,select wipe cache/dalvik,cache,and if you want clean install select also data and system.Press wipe.Go to home screen of TWRP and select format data,type"yes" wipe data.Now go to home TWRP and press install.Choose location SDcard and select downloaded rom ,swipe right.The rom Will be installed.wait to reboot till it says finished. Reboot system.Wait 5 min..rom is now flashed.If You want root also installatie supersu zip with TWRP. Go to TWRP choose install and select supersu zip file (here on forum)wipe cache, dalvik cache.reboot. Now your rom is rooted