Downgrade from Indesign CC 2017 to CC 2015

I'm using the Blurb Book Creator Plug-in. It worked in ID CC 2015, but I can't get it installed in ID CC 2017.

So my only option is to downgrade: remove the 2017 version and reinstall the 2015 version.

And there is where it goes wrong.

So far it was easy to reinstall older versions from the Cloud app, but the instructions to do this don't work anymore.

Where can I find the InDesign CC 2015 Install button now??