If you need to change an off day permanently, then you need to go to the Calendar Setup, Calendar ID, and change the Work Pattern or Leave Pattern.
To update an off day only in this specific year in the work calendar or leave calendar, please follow the steps below;
Work Calendar
Go to Core, Setup, Calendar Setup.
Select the calendar ID you use, for example, 5 Days Week Cal.
Select the Work Calendar and click the off day [eg. 12 Jan].
Right-click and select O.
Leave Calendar
Go to Core, Setup, Calendar Setup.
Select the calendar ID you use, for example, 5 Days Week Cal.
Select the Leave Calendar and click the off day [eg. 12 Jan].
Right-click and select O.