You can change the cheque/payment number and transaction type when editing. You cannot change the Bank account when editing the header if there are any detail lines.
Reconciled payments may be edited - but only the ledger account and description may be changed.
To select an existing cheque, click the button beside the Number field. If the form is in browse mode you may select a cheque from the form.
WARNING - If you created a cheque for the wrong bank account, it will have the wrong number, unless you entered it manually. If it has the wrong cheque number, and there are cheque details, you must delete and re-enter it.