You can compare the model opened in the COMSOL Desktop with a model version from the The Model Manager Window or 
The Databases Tree by selecting the version and clicking 
Compare (

) in the 
Item section of the 
Home toolbar. You can compare two model versions with each other by selecting two versions and clicking 
Compare (

). You can also click 
Compare from the toolbars of other windows that show model versions in the Model Manager workspace.