Only System Admins, or Full Users with the specific module "admin" role can access and edit the Form Definition.
To open the Main Form Definition simply click the Cogwheel icon at the top right of the screen in any record.
Note: This will only change the Form for the sub-module you opened the Form Definition from. For example, I opened the Form Definition in Quiz sub-module under the Training module. No other sub-module in Training is changed, only the form for Quiz.