A supplier shows up with 8 boxes when you ordered 10, or 10 arrive but 2 come broken, or a product you never even ordered shows up.
It's one of the most common situations in any business that buys merchandise, and the good news is that Odoo has a flow designed for exactly this: it's not about "correcting" the purchase order by hand, but about recording the reality of what came in.
The correct flow: receive what actually arrived
When you go to the receipt (Inventory > Receipts) and open the receipt document for that purchase order, Odoo shows you the ordered quantity on each line. There you don't have to force the number to match what you ordered: you simply edit the quantity to what actually arrived. If you ordered 10 units and 8 arrived, you enter 8 and validate. The system understands that 2 units remain pending and leaves them open on the purchase order for a later delivery (a "backorder").
This is key: you must not edit the original purchase order. The purchase order is the contract/agreement with the supplier; the receipt document is the physical reality. Mixing the two is the most common mistake I see, and it ends up creating stock mismatches that are very hard to trace later.
What if more arrives than ordered?
Same mechanism: if you ordered 10 and 12 arrived, you edit the quantity to 12 on the receipt. That creates a difference you'll have to resolve with the supplier (do they invoice you for the 12, or do you return 2?), but at least your physical stock will reflect what you actually have in the warehouse, which is what matters so you don't sell something that doesn't exist.
What if it arrived damaged?
Here there's a decision: if you're going to return the damaged items, don't receive them as good. You receive the sound quantity, and for the damaged product you do a direct return from that same receipt (Odoo gives you the "Return" option on the already-validated document), which automatically generates the reverse movement back to the supplier.
The impact on invoicing
This is what makes it worth doing right: if you have the option to invoice based on received quantity enabled (instead of ordered quantity), Odoo automatically builds the supplier invoice only for what actually came in. You avoid the "you overcharged me" fight because the system already controls it for you.