you can download the newest version of every theme through ThemeForest, you'll find a download section somewhere next to your name. Here you'll be able to find what you've purchased and the newest download version.
When updating, modifications you've made to the php will disappear. The shouldn't overwrite the custom.css. It's advised you keep a backup of all files you've modified. With the new theme download you'll find a list of updated files, you can just overwrite your old files only with the files that were modified in the new version to save you some time. If you've made any modifications earlier to the files that were updated you have to add those in again.