This marks the previous build as recommended

groovy:
import com.pmease.quickbuild.model.Build
import com.pmease.quickbuild.model.Build.Status

def lastBuild = build.getPrevious()
lastBuild.setStatus(Build.Status.RECOMMENDED)
system.buildManager.save(lastBuild)

However, this does not work with a currently running build,
because the status is changed after the build is finished

This can be done by running below in post-build script which can be defined in advanced setting of a configuration:

groovy:
import com.pmease.quickbuild.model.Build
import com.pmease.quickbuild.model.Build.Status

build.setStatus(Build.Status.RECOMMENDED)
system.buildManager.save(build)