Short Answer: It depends.
Longer Answer: Whether or not you may modify the amount claimed as due on your mechanics’ lien depends on several factors. If you are still within the time period where you would be allowed to file a valid mechanics’ lien originally, then yes, you are able to modify the amount of your lien. This may be accomplished by a modification of the original lien, or by filing a brand new lien with the correct amount depending on the laws of your state. If you are outside that time period – the answer is tricky.
Generally once you are outside the time period in which you could file a valid mechanics’ lien you are not able to adjust the amount due to a higher amount. This makes sense; if you did more work, and are owed more money for that work, you should be able to file a valid original lien to secure those funds. If you have not done more work, th amount should have been listed on the original lien. Also once you are outside that time period, you may or may not be able to adjust the amount of the lien down, at least without serious repercussions. If you attempt to modify the lien amount after the period in which to file a lien has expired, you run the risk of invalidating your lien by changing the filing date: just like an original lien, if the filing date is too late, the lien is invalid. You may be able to partially release the lien to modify the amount owed, but caution is needed here as well.