It's either the Category name - which you can edit in Site administration > Courses > Manage courses and categories
or more likely, it's the language string for "Courses" (core/courses), which you can edit in Site administration > Language > Language customisation - select the core/moodle.php language file and tick the box for "Customised only" and you should see your alternative text. Delete the customised text and scroll to the bottom and click "Save changes to the language pack" and then click on "Continue"
or more likely, it's the language string for "Courses" (core/courses), which you can edit in Site administration > Language > Language customisation - select the core/moodle.php language file and tick the box for "Customised only" and you should see your alternative text. Delete the customised text and scroll to the bottom and click "Save changes to the language pack" and then click on "Continue"