There should be a little 'edit' icon in the top right corner of your post. If it's not there please let me know and I'll check the permissions for you.
on another note , i edited an ad yesterday to reflect changes , but i noticed it didnt move from its current page 3 to the top of the classifieds on page 1. unless a potential customer went through every page it may not be noticed that i had made changes .. i rectified by posting a new ad .
If you make changes to your advert it won't effect the posted date therefore it will stay where it is on page 3. If you want the post moved back to the top you can either bump the topic or post a reply on your advert to say you've made some changes.