Open the entity and select the obligation. At the top, you find its general information and its linked registration. Further down, Filing periodicity shows the filing period, the frequency and the deadline rules, followed by the periods table with one row per period.
Correct the calendar
The filing period sets the calendar the periods follow: the calendar year or a custom tax year. Select the pencil next to it to correct it. Only do this when the original setup was wrong, because all periods follow this calendar.
Review or change the deadlines
Select the pencil on the periodicity. You see the filing due date rule, relative to the end of the period, and the calculation examples, so you can check the resulting dates before you save. The payment due date and the prepayments are in the same dialog.
When the frequency changes from a certain date
For example from monthly to quarterly next year: don't edit the existing periodicity, because that would change the earlier periods too. Select Add new periodicity instead, choose the new frequency and the date it starts. The earlier periods keep their own frequency.
Change one period only
For example when the tax office extended a due date: select Edit period on its row, turn on Override, set the new date and add the reason, so everyone can see why it differs.
Use Columns and Years to choose what the periods table shows, and the arrow at the top right to open it full screen.