In order to edit a piglet sale access:
Management > Phase the sale was entered > data entry > sales
On this screen you can filter according to the data entry.
To edit a sale, check the column action, pointing you mouse to the pencil to edit
It will open a screen where you can edit the following data: Date, amount, average weight, sale price, client and ATG (marked in red) After you edit the cahnged you need, just click on save and close.