If you need to delete multiple lines at once from a purchase order you can do this from the other functions menu
First open the purchase order you would like to delete the lines from
Second select all the lines you would like to delete and open the other functions menu, select 'delete selected rows'
You can also do this by clicking the delete text to the left of the line you wish to remove, you can do this for all lines you wish to delete and then click save under the table.
