If you want to edit an already approved delivery note it must be done in the following way. 

Find the delivery note that you want to edit in. Copy all the IDs of the animals on the delivery note by pressing Ctrl + C on your keyboard. 

Go to the tab "Delivery agreement" and enter the same information as there is on the delivery note with mistakes in it. When this is done click on "Create delivery note". 

On this delivery note you insert the IDs you copied by pressing Ctrl + V on your keyboard. The program will now show a message "This animal already had an exit" for all animals, press Enter on your keyboard. 

Make the changes needed and then you can press "Approve delivery note". The delivery note with a mistake will still be shown in the list, but it is possible to send the other one. In addition this, it is possible to add the column "Remark", where it is possible to put a note "Cancelled" or something similar.