In order to update your existing KNIME 2.1.2 installation go to "File->Update KNIME…" or "Help->Software Updates…". In the dialog that appears select the second tab named "Available Software", click the "Add Site…" button and enter the following URL:
http://reporting.knime.com/update/2.1/

Select both listed features KNIME Report Designer Feature and KNIME Reporting Runtime Feature. Follow the installation wizard and restart your workbench.
Alternatively, you can download an archived update site for offline installation.