In order to update your existing KNIME 2.2 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/2.2/

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.