Once a Sales Order has been placed in the B2B Store, no edits or amendments can be made to the Sales Order via the B2B Store. 

However, if the Sales Order has imported to Unleashed and has not yet been invoiced or dispatched (is not completed), an Unleashed User with the appropriate permissions can make any necessary adjustments in Unleashed directly. Any amendments made to the Sales Order in Unleashed will sync back to update the Order History in the B2B Store. 

For more details, see How to edit a Sales Order.