Solution
First, generate an overview of the entered data.
Go to '2. Overviews > 2.1. Sows > 2.1.10. Piglet mortality up to weaning'.
Select 'Detail lines' under 'Column layout'.
You can check the entered data in this overview.
Incorrect entries can be modified via:
'1. Enter/Modify > 1.1. Sows > 1.1.5 Mating/Farrowing/Weaning data (individual)'
Click 'Search' (or press F3) and locate the relevant sow.
Go to the 'F -6 Farrowing/Weaning' tab and correct the erroneous entry.