I got a great review from a guest; but no stars were checked. I contacted them and they said they thought they had checked 5 stars but must have done something wrong.
1. How can they now enter the 5 stars? (It has been about a week and I have already sent my review of them).
2. If it can't be changed, what effect does this have on my rating score?
Unfortunately reviews cannot be changed once posted unless there are content policy violations. A guest not ticking a star is not a violation. You will need to move on from this experience. More reviews and stars are in your future!
As for it counting against you, it does not, but it also does not count for you either. Airbnb's computer algorythm monitors the star checks and no checks are not computed in the tally.