Hello @Teresa473
Here is an Airbnb link that might help in understanding how to change dates for a reservation but the best thing would be to contact your host first and explain your reasons for wanting to change the dates.
https://www.airbnb.com/help/article/913
For you not to incur the 50% cancellation penalty, you must not cancel this reservation or ask the host to cancel and this must be an amendment to your current reservation that the host has to agree to.
By reading the link, it looks like you can make a "test" with the new dates and you should see if there is any extra money owed before pressing the send to host request.
Let us know how it worked out for you,
All the best
Joëlle