Rear camera rubber flap is torn

I have an iphone 5s with a torn rubber flap. Previous owner ripped it where it attaches/reattaches. I google it and see that replacements can be purchased but how do you install it? ( http://www.lunacommerce.com/3647-rubber-...) I am assuming this flap is necessary. The current rear camera is not working, but I have a replacement for it. Thanks