If a transaction has been incorrectly posted you'll want to make an adjustment to fix it. There are two ways this can be done.
Every page within a job (except those already displaying double entry journals) provide the option to add an adjustment.
A blank journal adjustment screen will display enabling you to enter a description, date, comment, account and amounts. You can add as many lines as you like and can even add a new chart of account code on the fly.
Alternatively if you drill into an incorrectly entered journal posting then you can just adjust the fields directly within the transaction. Save once the changes are complete.