Dropbox has started testing a new Windows Installer (offline version is only available at the moment) that has helper for updates included. Means, if you install this Dropbox test build, a new process ‘DropboxUpdate.exe’ runs from the location  ‘C:\Users\username\AppData\Local\Dropbox\Update’ and upgrades the build to new one automatically, when available. This process was configured to run on startup of Windows, which you can disable at any time.

Dropboxupdate.exe process

How to switch from Dropbox experimental to Stable version and Vice Versa

Other changes/new features Experimental build v3.5.54 brings are