The lien is not void unless you willfully overstated it, or made with the intent to defraud. (Civil Code Section 8422). If it is less than ninety days from completion or less than 60 days from the recording of a notice of completion, you could record an amended lien for the correct amount. Keep in mind that you will need to sue to enforce the lien within ninety days of the date of recording.
Disclaimer: The materials provided below are informational and should not be relied upon as legal advice.
If the amount is incorrect, you may need to file an amended notice of lien. Mechanic's lien law is somewhat technical; in order to have a valid lien, you need to comply with proper service of notice of lien, timely initiate legal action after recording lien, timely record lis pendens, etc. I suggest you immediately retain an attorney to help you through this process.
Rather than amending your mechanic's lien, and chancing that the amended lien is recorded too late, you may be able to record a partial release of the initial mechanic's lien, releasing only the amount that isn't actually owed. You would still need to file suit to enforce the mechanic's lien within the short period of time allowed following your initial lien.
The deadlines for mechanic's liens and enforcement are short and the state statutes with which you must comply are lengthy. I have seen far too many clients lose their rights to enforce a mechanic's lien because they thought they could do it on their own, and sought legal help too late. Of course, even if you cannot enforce your mechanic's lien, there are often other claims that you can make to attempt to recover.