To edit a transction make sure the relevant transction is selected first. It will be if you have activated the maintenance from from the transactions view form.
When editing transactions the transaction type cannot be changed. The type determines how the balances are updated "on the fly"
All other visible fields may be edited. When editing, or adding, the Details button is disabled. You must save, or cancel, before you can change details. You can add details at any time. Provided the transaction has not been reconciled or G.S.T. imputation completed.