The most common reason for editing a system account is a change of bank. One of the system accounts is the "Electronic Payments" System Account. This is the account that your invoices look for when displaying the Bank Account for Clients to pay into on your invoices (if you are using generic fields). If you change Bank you will need to direct this to the new bank account.
Go to the system accounts list and select the Edit button to the right of the account. Re link to the new account in your accounts list.
While there may be other scenarios where an account edit is required; it's not common once the system is being used.
Go to Admin > Accounting System Accounts > Select System Account > Edit Details > Save