The quantity showing in Deliveries to update is from any Sales Orders that have had the Delivery Note printed but NOT updated.
There are two ways to clear that quantity, the option chosen depends on whether the Delivery should have been printed.
Option 1 - If Delivery Note should have been printed and needs updating
Via the Sales Order Process Batch window
Flag Sales Order as Yes
Make sure you have the Update Delivery box ticked.
Then Green Tick to process
Option 2 - If the Delivery Note should not have been printed
Via the Sales Order Process Batch window
Flag Sales Order as Yes
Then click on Reset Print Status icon (blue swish arrow).