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.