If you are using the Cost of Sales method for your Stock accounting and need to adjust an entry, you will find that you cannot edit existing entries. This is because the period Cost of Sales Adjustment amounts reported in your P&L are calculated at the time each Stock transaction is entered and these are based on older values such as prior opening balances. This means that any changed values would need to cascade forward to all later transactions - and so it is not possible to edit existing entries.
OK, so what do I do if I do need to edit a transaction?
The way to do this is to delete your transactions and re-enter them. Start with the most recent transaction and repeat until your have deleted the one that needs adjusting. Re-enter this with the correct value and then all other transactions to get back up to date (hopefully not too many!). It is a good idea to take a screenshot of the Stock transactions before removing them so you can re-enter them correctly.
(You may also need to move your Financial Year-End backwards if the Stock Transactions are in a prior year).