You should be able to update directly to the new version using the Syn3 Updater. Just select the current version on your device, 3.4.20196, and the version you want, 3.4.20237, and make sure you select NON-NAV, or NO MAPS. Run the update in the vehicle.
However, having a NON-NAV unit, you might find the update may fail. If this is the case, don’t panic. Go to this tutorial and perform the Gracenotes removal, then update the unit again.
Read this and follow the instructions for Gracenotes removal.