They did release a compatible version not long after 3.9.0.55 came out. You'll need to upgrade to that first, then it will let you update the plugin.
I'm looking to update our Dell OME to 3.9.0.55 but appears the Update Manager is not compatible.   Is there a new version of the update manager on the horizon that will be released ?   [IMAGE]  
Update to 3.9.0.55 - Update Manager Not Compatible