Once you reply to a posting, you will get an 'Edit' button on the bottom of the window next to the 'Quote' button which will allow you to edit your response.
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Tom and Margaret
2014 Berkshire 390bh-60
If it's an old ad, the edit period has likely expired. You can ask a mod to change your ad (which we will happily do) or you can post an update to the thread.