Snippet to modify an external tool on Canvas via JS.

canvas_edit_external_tool.js

// Query to GET a list of external tools, filter it for your tool, and then PUT it's changes
// Need to change the <course_id>, add your filtering criteria, and
// customize your data attributes for the tool.

$.get('https://canvas.instructure.com/api/v1/courses/<course_id>/external_tools/', {}, function(result) {
    $.each(data, function(i, item) {
        if (item /* matches some criteria, maybe the name? */) {
            $.ajax({ url: 'https://canvas.instructure.com/api/v1/courses/<course_id>/external_tools/'+item.id, 
                 type: 'put',
                 data: {'course_navigation[enabled]': 'true',
                        'user_navigation[text]': 'Something Cool',
                        'user_navigation[url]': 'https://example.com/ims/lti/user_endpoint'}
            })
        }
    }
});