In Qb POS, you can edit the assembly item to change the qty of the component items.

Pull up the item in the items list, and click Assembly in the right pane preview, then edit assembly.

That will allow you to change the items going forward.

You cannot however, change the ones already sold.

Transaction in QB POS are like a instant snap shot. Once the picture is taken, you cant change it. it is pretty much carved in stone.

You could correct the item, and do an exchange on the sales to get the inventory corrected. You can override the date on the exchange to make it the same day the original sale was done, so, exchange the old assembly for the new one..

As always, make a backup prior to doing anything. That way if things go wrong, you can restore the backup and you are no worse than you are now.