You can change a confirmed order — quantities, prices, dates. What you cannot do is change it below what has already happened.
Make a change
Open the purchase order.
Click Edit in the top right.
Change the quantity, unit cost or dates.
Click Save.
The line items look like plain text until you click Edit — that is normal, not a permissions problem.
What you cannot change
You cannot reduce a quantity below what has already been received. If 40 of 100 have arrived, the lowest you can set that line to is 40. Brahmin Solutions refuses anything lower.
You cannot reduce quantities at all if the order has more than one bill. You will see: "Cannot reduce quantity as multiple bills exist. Please adjust bill quantities first." Change the bills first, then the order.
A price change reaches the bills too
Changing a price changes the bills too. If a bill already exists against this order, editing the unit cost rewrites that bill to match — including a bill you have already entered from the vendor's invoice.
If the bill has been paid or sent to your accounting system, check with whoever handles your books before changing the price.
Reducing a quantity also reduces a single bill. Increasing it does not — the extra is billed on a new bill instead.
How to know it worked
The line shows your new values, and the order total at the bottom has recalculated. If a bill exists, open the Bills tab and check it says what you expect.