A final judgment may be set aside in limited circumstances, on a motion made within the applicable period.
Grounds with a short outer limit, usually one year: mistake, inadvertence, surprise or excusable neglect; newly discovered evidence that could not have been found with reasonable diligence in time for a new trial motion; and fraud, misrepresentation or misconduct by an opposing party.
Grounds without that limit, subject only to a reasonable time requirement: the judgment is void, typically for lack of jurisdiction or a due process defect; the judgment has been satisfied, released or discharged, or a prior judgment on which it was based has been reversed; and any other reason justifying relief, a residual clause reserved for extraordinary circumstances and not available where another ground fits.
Default judgments attract the most of this practice. Courts weigh whether the default was wilful, whether the movant has a meritorious defence, and whether the other party would be prejudiced — with a strong preference for deciding cases on the merits.
Independent actions and fraud on the court exist for the rarest cases and require conduct that defiles the court itself, not merely fraud between the parties.
Move promptly. Delay is the most common reason relief is denied.