The App Center has tabs for upgrading currently-installed apps and browsing apps which are currently available (not that some apps are not maintained anymore to be recognized by a not maintained label).
To upgrade an existing app, simply click the Upgrade app button and open a vote to upgrade the app if you do not have direct permission to upgrade the app.
App installed on the DAO
Example of App details (Agent installed on the DAO)
Apps that can be installed on the DAO