I just took dilvery of this lens it it has a smudge on the inside rear element, obviously I'm not happy about this but as it came from America and I'm in the UK I don't really want the hassle of sending it back, is the inside rear element hard to get to on that lens?
Usually just a spanner ring around the element. unscrew, element falls into your hand or onto the floor.
You need to recognize which side was in/out, Use a piece of masking tape on the outside & you have it marked and something to hold it with.