How can I patch "R Services (In-Database")?

I've just installed "R Services (In-Database)" onto an existing, updated SQL instance but I can't get the launchpad service to start. Looking online, this seems to be related to the issue:

    8. Launchpad services fails to start if the version is different from the R version

https://docs.microsoft.com/en-us/sql/machine-learning/troubleshooting/known-issues-for-sql-server-machine-learning-services?view=sql-server-2016

Checking the details, I can see that the version of "Launchpad.exe" (13.0.1601.5 - RTM) is different from "sqldk.dll" (13.0.5820.21 - SP2/CU13) but when I run the SP2/CU13 executable, all options for upgrade are greyed-out and clicking on the "R Services (In-Database)" entry it reports "Installed".

How can I update the R Services components which need it?