The Premium Plus themes on the Marketplace get maintained and updated in line with Zendesk's evolution. More info about these changes can be found in the "Release Notes for Themes".
Auto-updating your Theme
When you install the Theme, you can select to update the Theme automatically.
After you confirm, all new releases for the corresponding theme will be updated.
If you didn't mark the auto-update feature, you can still update your version to the latest one.
Customisation of the Theme
If you bought the developer license, you can access the Code of the Theme.
By changing the code, your theme will not get updated automatically.
In itself, this is not a problem. Note that some new features might not appear in your Helpcenter. If this causes an issue, you can reinstall the purchased Theme.
Notification of a new Theme version
Zendesk will indicate that there is a new version available of the Theme you purchased.
Reinstall the Marketplace App
At any given moment you can check your purchased Themes.
From here, you can see both the trials you tested as the purchased Themes.
By right-clicking on the purchased Theme, you can add it again to the Theme library in its pristine state. The changes you made on the installed theme will not be applied or copied over, to the "latest version" you just downloaded. You will need to re-apply these changes to this newly downloaded version.
Therefore we recommend using Github, enabling you to merge your changes with those from the updated Theme (Ask your developer or IT service how this works)