Best way is to flash the stock FULL rom if you have a copy it. How did you root it and what root app got installed? Is it SuperSU? If not, then you might want to change the recovery to TWRP and flash SuperSU's flashable zip file. Then you'll have SuperSU app installed, you can use the settings of it to unroot (full).
If you've done a lot of changes while your phone is rooted, then really best way is to flash the full stock rom. Or try out MM: http://community.flash3c.com/t/how-to-use-flashtool-to-flash-marshmallow/5855?u=seedr