Hi there, well I do fixed my brother’s iPhone 7 Plus Lens doing this.
You need to open the iPhone, remove all internals (so nothing gets damaged). It looks like the lens is attatched from inside to outside, it has a metal braket inside that holds the lens in place.
It is not possible to replace it from the outside without opening the iPhone.
After opening the iPhone and removing the braket, you can remove the extra adhesive that has the lens itself.
Remove the old broken lens and add a new one. The metal braket will be unusable at this point, so make sure to use good quality adhesive.
Pur all the internals inside the iPhone and you are good to go!