Here are two sample scripts along with an API to provide automatic updates for plugins and themes you host on your own server.
/api you’ll find
index.php which processes all the update requests. You should place this in something like
http://updates.example.com and update
/theme/portfolio-racer/inc/updates.php accordingly. If you activate these sample plugins without changing API URL, updates will be checked against my test server. If you decide to update, both plugin and theme will be replaced with exactly the same version of each.