In order to set variable working hours in workweek you have to change workweek scheme type to Separate work hours for each day. Then
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 select get customized working hours for each day (1):