I would like to change my dates of booking from 3- 9 Aug 2018 to 3-7 Aug 2018. And please re calculate the amount to pay. How do I go about doing it?
Try contacting your accommodation provider.
These are discussion forums. Have you booked via TA?
Possibly go back to whoever you are booked with, (I assume it to be a hotel in Windsor? ) And ask them. This is a public discussion forum, not a booking site?