Web modeler is discontinued so this feature is no longer available in 9.21. So make a branch of a 9.15 version and merge it there and commit it.
Ok. I figured out the answer.
On Mendix Pro 9.21,
Version Control → Branch Line Manager → Select on the Main Line in the “Branch Line Panel” and click the “New” Button.
A new window called “Create Branch Line” will pop up.
In this window, one does need to click on the SELECT button to select a revision version and then enter a desired branch name.
Once the new branch is created. the older version can then be loaded into the Pro 9.21.