UPR is build using Tycho, so Maven 3 has to be installed. The main build script for the standardized profiles is located in project "standardized/build/org.eclipse.upr.build". Execute e.g. "mvn clean install" in that directory to build all projects.
The UPR update site will be available in directory "standardized/build/org.eclipse.upr.profiles/target" after a successful build.