UML includes two elements that are used to model Events. The first element is the Send Event. This element models the generation of a stimulus in the system and the passing of that stimulus to other elements, either within the system or external to the system.
The second element is the Receive Event, which is depicted as a rectangle with a recessed 'V' on the left side. This element indicates that an event occurs in the system due to some external or internal stimulus. Typically this invokes further activities and processing.
If you should select the wrong type of event, or otherwise want to change the type, right-click on the Event and select the Advanced | Make Sender or Advanced | Make Receiver context menu option, as appropriate.