I can't find the specific procedure for doing this. I've tried running the upgrade wizard from the SQL install utility, and the upgrade goes fine. However, the ServerUpgradeHistory table for the Service Application still shows a max version of 162, which is the same as SQL 2012. I've tried deleting and recreating the service app and SSRS databases, with the same result.
Details: SharePoint 2013 on-prem. Had SSRS 2012 in SharePoint integrated mode. Trying to upgrade SSRS to 2014.