You can only reprint a label if it hasn't been manifested yet.
To adjust the order first:
Expand the order.
Edit the details you need to change (eg the total weight of the order).
Select the order and click Print Labels again.
A dialog box will pop up asking you if you want to Reprint or Update.
If you have changed the order select Update to make sure the label is regenerated with the new details.
Otherwise just select Reprint to generate a new label.