You can complete an already invoked activity in the Activities Execution view of the Workflow Execution perspective.
The Workflow Execution Perspective calls the completion method as defined in the application definition. To complete an activity, click the Complete icon in the toolbar of the general worklist view.

Figure: Complete an Activity
After the activity is completed, the Process Portal will try to find a succeeding activity of the same process instance the currently logged-in user is allowed to work on. If such an activity is found, it will be presented to the user immediately. If not, the standard view will be returned.