Many functions are based on sketches, such as Extrude and Revolve. You can also use sketches when choosing sections and boundaries for faces. To edit the sketch, you can either:
Right-click the sketch name on the Feature Tree. In the popup menu, select Edit Feature.
Select a face or edge of a feature based on a sketch, right-click and select Edit Sketch / Composite. This only works if the selected entity was created by one function only.
Right-click on the feature name (if the feature is based on a sketch) in the Feature Tree and select Edit Sketch / Composite.
You will enter the Sketcher. When you have finished making your changes, exit the Sketcher and the feature will be updated.