Dropbox and several other cloud platform providers support file version history. When diagrams.net auto-saves, a new version of your diagram is created.

  1. Open the diagram stored in Dropbox in the diagrams.net editor.
  2. Select File > Revision History from the menu.
  3. Preview the different versions of your diagram by clicking on the tabs on the left. 
  4. Click Restore to restore an older version of your diagram.