You can change these by going to the Business -> Opening Hours section. Scroll down to the Irregular Opening Hours section where you can create all the dates for the public holiday in question - these will overwrite the default hours you currently have set on those days.
Note, if you select Repeat it will implement these opening hour changes on these same dates each year - this would work for Christmas which is always on the 25th December, but not for Easter which is not a specific date.