How do I file an extension for this lien?

Unfortunately, the answer is, it depends. If this is a single family home, or you have previously extended the lien, you will need to bring a special proceeding in court to extend the lien. Further, because the deadline is rapidly approaching, you need to do this by order to show cause in order to get it in front of the judge more quickly. However, if it is not a single family home, or you have not previously filed an extension, it's a matter of filing a simple form.