How do I update to the latest version of the theme without undoing all my work in the previous version?
Download the latest version of the theme from your ThemeForest account, login and then click on “Downloads”.
Always backup your site and database before performing an update.
Now, assuming that you haven’t modified any of the theme’s files, you can simply drag and drop in your FTP client the latest unzipped version over the old one to overwrite the files. That way if you have uploaded any images under sliders, etc. they will not be deleted.
Now if you have modified files like CSS or some php files and you haven’t kept track of your changes then you can use some ‘diff’ tools to compare the two versions’ files and folders. That way you’d know exactly what files to update and where line by line. Otherwise you’ll loose your customizations. A couple of compare tools are ‘Beyond Compare’ (http://www.scootersoftware.com/) for PC and Changes (http://connectedflow.com/changes/) for MAC that even allow you to merge the changes into yours.