SQL Server Reporting Services 2017: Install Cumulative Update
We’ll periodically update the SSRS 2017 download/installer such that it’s always the latest “Cumulative Update.” If you have an earlier release of SSRS 2017 installed and you run the installer, it’ll prompt you to “Upgrade Reporting Services” to install the update.

The Reporting Services installer is separate from the SQL Server installer and SQL Server CU installers. We’re not currently releasing it through WSUS, but we’d love to hear more customer feedback on that topic.

Reference: SQL Server 2017 Reporting Services now generally available