In the past, we could run the SQL installer to do an in-place upgrade of SQL server including the SSRS instance to a newer version. As of 2017, SSRS is a separate install from SQL server, so this is no longer possible. In fact, if you do an in-place upgrade of SQL 2014 to 2017 for example, you will see a warning that SSRS will be uninstalled. Before proceeding with the SQL upgrade, follow these steps to upgrade the SSRS instance.
It’s not as easy as it was before, but Alexandre takes us through the step-by-step process and even includes some notes on how to roll back your upgrade attempt if necessary.