Right-click the Trim name in the Feature Tree, or select the Curve and then right-click. In the popup menu that appears, select Edit Feature.
The Feature Guide will appear, automatically open to Required Step 2. You now return to the same situation in which you created the Trim.
Make modifications in the current step, and / or click the icon for
Required Step 1 and make modifications
in that step. Press OK 
or Apply 
in the Feature Guide to complete the function.
Note: If your changes result in a geometric impossibility, or cause problems in subsequent features, you will be warned by the Conflict Solver.
See also: