Hello @Carol274,
The guest will get the rate that applied when they made the reservation. Your subsequent price changes will apply only to reservations made after the price change.
The same applies to any other changes you make to your listing e.g. change of max number of people, change of check in / check out times, change of cleaning fee etc.
When a guest makes a reservation request and the host accepts it, the guest is charged immediately and they can expect to find the listing as it was i.e. all amenities, check in/out times etc. as they were when the reservation was accepted.
Steve.