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, hovering over 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 and make changes you need, simpl click on save and close.