This depends on your business model. If your theme is free (or freemium, meaning people can download for free but unlock features when they pay you), you should post it into the wordpress.org repository. There’s a whole lot of rules you must stick to, which you can read about here. The upside of being in the repository is that they will take care of pushing notifications to your users once new versions are available.
If you want to run your own update server, you’re up to quite some work, because the WP installations of your users will not simply grant access to a third party computer trying to change files. Your theme must include an API that makes regular calls to your server to see if there is an update, notify the user, download the new theme files and then unpack them to the right directory. It’s beyond the scope of WPSE’s Q&A model to outline how this sizeable work is done, so you’ll have to look for a tutorial (example).
Note: do not take for granted that all your users will automatically receive the new version, because they may have disabled updates.