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// For example if you have a node with keyname foo and wanna add something to it's text.
$node = $PAGE->navbar->get('foo');
if ($node) {
    $node->text .= ' adding text';
}

// If you wanna change all nodes.
foreach ($PAGE->navbar->get_items() as $node) {
    /** @var navigation_node $node */
    $node;
    $node->text .= ' aaaa';
}