Hi Guys, I bought a used iphone 11 pro max, but it has android OS. I am not sure how to get it back to IOS. Please help me in this regard.

You did not buy an iPhone. You bought a cheap counterfeit, commonly referred to as an "iPhoney".

Return it and get your money back.

I am no expert but I think what you are suggesting is not possible.

i would return the phone and not use it.

You may have bought an iPhone knock-off from China. One of the tech sites had an article about this. It looks exactly like an iPhone but it's not. It ran an Android OS. It actually worked pretty well but obviously didn't work like an iPhone.

Just because the serial number is valid does not mean it belongs to the device you have in your hand.

The "14 days" only applies to devices purchased directly from Apple.

Apple does not sell "used" iPhones at all.

Where, exactly, did you buy the phone?

If it wasn’t jailbroken, it cannot have non-iOS apps on it. What do you see that makes you think an android app is on it?

but it has android OS. I am not sure how to get it back to IOS. Please help me in this regard.

How are you determining this?

If this is accurate, the device was jailbroken to do that.

Restoring it using iTunes "should" get you back to default, but there are no guarantees. It would spend on what was done to get Android on the device.