You edit an existing composition by selecting the Edit menu item and then clicking on Edit Composition. This will display a dialog that prompts you to select a composition to edit. Highlight the composition you want to edit and click OK, or double click on the composition name:
If you select a method composition, the Edit Method Composition dialog is shown; if you select a call change composition, the Edit Call Change Composition dialog is shown.
If you want to create a composition that is similar to an existing one, you can click Edit/Dupicate Composion, to produce "Copy of" the existing composition - then use Edit/Edit Composition to change the name and content of the copy to produce the new composition.