Override Save button in Default Dexterity Add form

Would it be possible to override the default 'SAVE 'button on (Generic) Dexterity Edit forms, so it only saves (and not 'leaves').
(And then change the 'Cancel' button to 'Exit')

If so, how should this be done ? Do I override the buttons or the Edit form ?

You would need to create a custom form based on the DefaultEditForm from plone.dexterity.browser.edit.py. Then you can customize the buttons and handlers.