You can edit event attributes in two ways: using the event's attribute table or using the Attributes dialog box.
As with editing the attributes of any feature, editing event attributes takes place within an edit session. Start an edit session by clicking Start Editing from the Editor menu on the Editor toolbar. Once you begin an edit session, you'll notice the Edit tool button 
next to the Options button on the table window, indicating the table can be edited. In addition, those fields that you can edit will have a white background color for the field heading.
About starting_and_stopping_an_edit_session
Make attribute changes by clicking a cell and typing a new value. If you make a mistake, you can easily undo the edit by clicking Undo from the Edit menu.
Editing attributes through the table window allows you to quickly make changes to several records at once. When you're editing the attributes of a specific event, you may find it more convenient to use the Attribute dialog box, which is accessed from the Editor toolbar. To use the Attributes dialog box with events, you must first add your events to ArcMap as a feature layer.
The Attributes dialog box has two sides. The left side lists the features you have selected. Features are listed by their primary display field and are grouped by layer name. The right side shows the attribute field names and their values.
About editing_values_in_a_table
About making_field_calculations
When you've completed your edits, you can save them and end the edit session.