If a holiday duration is greyed out and cannot be amended, as highlighted in the screenshot below, this indicates that the user was assigned to a work pattern during that period when the holiday is booked for.
When assigned to a work pattern, the duration information is picked up from the working hours specified in the work pattern. To override this, you would need to assign the user to a No work pattern with an effective date prior to the holiday date.
To do this:
go to the user's planner
click into their work pattern
from the drop down list, select No work pattern
set the effective date to at least one day before the old work pattern
For more information on how to remove historical work patterns, please go to the 'Deleting Historical Work Patterns' section of the below article: