A double-entry transaction is a transaction that appears in two accounts. One account is debited by an amount exactly equal to what the other is credited. Thus, all transactions are always transfers between two accounts. Since they always appear with a plus sign in one account, and a minus sign in the other, the total over all accounts will always be zero, and thus, balanced accounts are guaranteed.
Double-entry is already familiar to most people as a transfer from one bank account to another, where money is withdrawn from one and deposited in another. Far less familiar is the idea that double entry can be used to track income and expenses as well as bank transfers. See the "Income/Expense" help window for more information.
To change a double-entry transaction, edit it in any window in which it appears. Any changes made will be automatically reflected in both accounts and all windows displaying the transaction. Similarly, when a double-entry transaction is deleted, it is deleted from both accounts; balances are automatically recalculated for both accounts.
To change the transfer account, simply select a new account from the pull-down menu. When you record the transaction, it will automatically be selected from the old account, and inserted into the new account.