After two years of being a Conditional Permanent Resident a Green Card holder must file to Remove Conditions. See this Guide and Section 7 of the FAQ's for general information on removing conditions.
per the USCIS:
The I-751 is used for a conditional resident who obtained status through marriage to apply to remove the conditions on his or her residence.
additional references:
(links below may be broken due to USCIS website update)
General FAQ's for Removing Conditions
in Removing Conditions on Residency General Discussion
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