If you wish to limit the number of attempts allowed at a course this can be managed from the course availability settings.
Select MANAGE AVAILABILITY to access the attempt limit settings. The limit can be turned on and the default number of attempts can be set from this screen.
The settings from the course are applied when the user first starts the course. If you change the settings after that the user will still retain the settings they started the course with.
Limits per user
If you need to edit the number of attempts for an individual you can do this from the course report for that user (either select the user from a course report, or the course from a user report).
Selecting EDIT from this screen lets you add a specific number of attempts for this user, or edit the value that was set by default.