If the account is the correct one for that transaction:

  1. In Records, Accounts, open the account (if it does not exist, create a new account)
  2. Change the status to active
  3. Save and Close
  4. Post as usual

If the account is not correct:

  1. Find the transaction and open it
  2. Change the GL Distribution to the correct account
  3. Save and Close
  4. Post as usual