See: Why edit a Part?
Use the Part-Editor to edit the length and/or add sketch-elements to a Part or the Base-Part.
You can edit only one Part at a time.
To edit a different Part from the Part that is open in the Part-Editor:
1.Close the Part-Editor, to return to the Mechanism-Editor, and then,
2.Start the Part-Editor again to edit the different Part.
5-Methods to Start-the-Part-Editor
Graphic-Area : show sketch-elements, dimensions, Part-Axes
Contextual toolbars : to the left and right of the graphic-area
Name-tabs : the active name tab is the Mechanism-Editor to which the Part is a child.
Menus and Toolbars:
Geometry menu and toolbar to add sketch-elements.
Constraints menu and toolbar to add constraints to, or between, sketch-elements