Hi,

your question is :"how to modify  validated(Done) delivery order or draft/waiting/ready delivery order?"

Note : images from Odoo v16 but the behavior is the same in previous versions

for first option:

1. suppose you confirmed 100 units

Note : the line will appear in red color when there is no available quantities

there will be one record in valuation layer,but the entire quantity in inventory report will be -50

record in valuation layer:

inventory report:

2. go to delivered transfer (status : Done) and click on "UNLOCK" button, then edit previously "Done" quantity for 50.

Note: this will create another stock.valuation.layer record with updated quantity

the inventory report will be updated (no negative quantities for this product)

Note : this option will work if you do not want to update sales order

the second option would be to create "RETURN" and update Sales Order: (which I recommend)

1. same as first option
2. on delivery order click "RETURN", then update quantities to "50" and keep update sales order checked.

3. the quantity will be updated on sales order, and return transfer will be created

Note : the return transfer will update negative quantities in inventory.

Hope this will help.