To edit a pre-existing activity, a copy of the activity must be placed inside one of your libraries. Find the activity in your library, then click the edit button on the activity. This will open the activity editor.
If you do not see the edit button, you do not have edit access to that activity. To gain edit access, preview the activity, then click “I want to…” in the upper right corner. Click “Add to My Library” and select the library designation. An editable copy will be placed in your library.