The 5 steps guide

Basically speaking:

1. Pull the new artifacts from http://maven.ontotext.com/content/repositories/publishing-releases
2. Stop your Tomcat (or other webapp container) and replace the recommendations-web.war (also delete the unpacked war directory)
3. Go to your solr.home (see where you set it here) and delete everything except for the data directory
4. Unpack recommendations-solr-core.zip in solr.home
5. Start Tomcat