To specify non-standard work hours for activities:
Navigate to Table
Navigate to the activity that you want to change work hours for (You may have to expand the Group if the activity is inside a Group)
Find the WRK HRS column and change the work hours as needed
Easily copy and paste the work hours to other activities using <CTRL>+C and <CTRL>+V
Press 'Schedule' on top right to recalculate the total resource hours if the schedule is resource loaded
(Please note, changing work hours will not change the duration of the activities).
Tip: Use Filters and Custom Tags to filter all activities with non-standard work hours and change them by using copy and paste functionality in one go.
For more information on Filters click here and for Custom Tags read this article.