Once a settlement agreement has been signed, it's generally too late to change your mind. You're stuck with the outcome of the agreement, even if you have regrets later. However, if you're unhappiness is the result of something shady that happened during the proceedings, here are two ways you can get the settlement contract invalidated.
Show the Defendant Behaved Fraudulently
Possibly your best option for getting a settlement agreement invalidated is to show the defendant committed some type of fraud during the negotiation process.