Every time a DMN diagram is saved, a new diagram version is created.
Quickly browse through older versions by clicking on the current version in the header bar.
Milestone versions can be managed via "File" → "Version Control".
The corresponding dialog allows to
mark versions as milestones
see comments attached to diagram versions (to enable commented saving, go to "Settings" and enable "Enter changelog when saving the diagram").
load older versions. When you save your changes, a new version will be created and saved as the latest diagram version.