I am unsure of which mesh you are referring too but here goes both
# The one I think your referring to, the mesh on the bottom of the phone. To replace this you would have to replace the whole casing of the phone which you probably dont want to do. If the only problem is that they are dirty, take an old toothbrush and compressed air can and scrub with toothbrush and remove excess with air. Repeat until clean
# The second mesh is on the top of your screen and to replace it you would need to (I believe) replace the screen. This is also pretty expensive so what I would do is just do the same thing as what was listed in number 1 just take greater care not to damage the screen or selfie camera(and lose face ID)