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Sorry, but there is no way to tighten it. You have to replace it. It is common for the plastic to crack and the visor falls down. You can try and put a self taping screw in it, but it will look terrible and not work properly.
Sorry. Here is how to remove the header panel.
1. Disconnect the negative battery cable.
2. Remove the striker.
3. Remove the sunvisor.
4. Remove the interior light. (See INTERIOR LIGHT REMOVAL/INSTALLATION.)
5. Pull the front header trim toward you, then disengage the clips A, B and pin C from the body.
6. Install in the reverse order of removal.