In order to set variable working hours in workweek you have to change workweek scheme type to Separate work hours for each day.

To use this feature, access "Management" menu (1):

Then head to "Workweek schemes" (2):

Edit desired workweek scheme (3):

Press "Edit" button located under "Edit scheme type" (4):

From a drop-down, please choose "Separate hours for each day" (5) and approve change by pressing "Edit" (6):

Then, after making necessary amendments in workweek scheme edition, approved by pressing "Update workdays" (7) it will be possible to get customized working hours for each day: