i purchased an iphone 6 plus from a wholesaler in china at the factory, it looks like it is the ios system but is running on android when I checked all the components, how do i wipe the phone so i can install the ios system?

I purchased an iphone 6 plus from an authorized seller from china factory unlocked, 128gig, anyway it looks like my other iphone screens and all but when i go into the internet and some of the options are not available to use, and when i reset the phone it shows the android symbol, i put my sim from my other phone in this phone and had it get my email, and with my email it comes up that i have signed into my email with an android phone, i want to wipe this phone and return it to the right operating system, how can i do that, i have been trying to research this for days and have come up empty, can you help plz??