I ran the update, but I still see the old version of the plugin in my DAW

The first thing you try is to restart/relaunch your DAW if you haven't already. Some DAWs cache plugins in memory and removing the plugin won't be enough to get the latest version. 

This issue might come from the fact that you are using Windows and Windows doesn't have a single, dedicated location for VST Plugins like on a Mac.

The problem might be use to the fact that you are installing the updated VST in a different location than you DAW looks for plugins. 

Go in to your DAW settings and look where it is set for VST plugins: 

• Ableton Live, this is in Preferences->Plug-ins or (Preferences -> File/Folder in older Live version). Check the "VST Custom Folder" 
• in FL Studio, this setting is in the plugin manager (Options->Plugin Manager) 
• For most other DAWs, this VST folder path is in the Preferences. 

The most effective way to make sure everything is clean is to go to the above location(s) in Windows Explorer. 

• Delete the plugin file (e.g. Serum.dll / Serum_x64.dll) from any and all folders your DAW looks to for plugins. 
• Rescan in your DAW (or restart it) and verify that Serum is no longer available. 
• Then run the update installer to the location where the plugin file(s) used to reside at.