Just because you saved your journal doesn't mean your words are set in stone! To edit a journal entry you've already saved, visit your journal and click on the "edit" link in the top right corner of the journal entry you'd like to change. You will be taken back to the edit screen where you can make your changes.
Note about changing privacy settings: If you want to make your journal entry more private after you've already saved it, please keep in mind that it may take a few hours for the privacy setting to take effect. Also, Google may have a cached version of your journal entry if at one point you made it public, and it may take some time for that cached version to disappear from Google even after you make your journal private. These will disappear from search engines over time, but just know that it's not immediate. To guarantee your journal entry will not be seen by anyone you don't want to, always double check the Privacy settings of anything you upload.