To execute the default action for a file, double-click on the file. For example, the default action for plain text documents is to display the file in a text viewer. In this case, you can double-click on the file to display the file in a text viewer.
You can set your file manager preferences so that you click once on a file to execute the default action. For more information, see Section 6.12.2 ― Behavior Preferences.