In reference to the patent: US 6499671 B1
What action is required to reinstate this patent?
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This link describes How to Reinstate a Patent
I'm guessing it's more than 24 months lapsed, so see the second paragraph below. My other guess is that you're trying to figure out how much you'd owe. I didn't see how to determine that (aside from the fee schedule displayed, but then you need to know what is already paid). Near the end of the page they list a phone number (also shown below).
From the linked page:
If a maintenance fee has not been paid in a timely manner and the owner of the patent wants to get the patent rights reinstated, a petition and proper fees are required.
If the date of expiration is more than 24 months, file a petition by submitting the completed form SB/66 and paying the proper fees; there are significant processing delays and the filing does not guarantee a granted petition. Contact the Office of Petitions at 571-272-3282 for assistance on filing a petition to accept unintentionally delayed payment of the maintenance fee.
If you need additional assistance, please contact the Maintenance Fee Branch by telephone at 571-272-6500