You can enter the Assembly module at any time during an Abaqus/CAE session by clicking Assembly in the Module list located in the context bar. The Instance, Constraint, Feature, and Tools menus appear on the main menu bar.
To exit the Assembly module, select any other module from the Module list. You need not save your assembly before exiting the module; it will be saved automatically when you save the entire model by selecting FileSave or FileSave As from the main menu bar.