When upgrading clients there are 2 scenarios: manually added clients and provisioned clients.
Step 1 - Update Repositories
Manually Added Clients
Update the Katello client release packages:
Select your Operating System:
If the katello-agent was setup during provisioning from a locally synced repository then you will need to go through some initial setup to add the 3.15 client repositories to your Katello for each version needed. After you create the new repositories, they will then need to be added to the relevant content view(s) and the older versions removed. At this point, a new version of the content view can be published and promoted to the appropriate environments. Once the new package is available the clients can be updated following the next steps.