Carolyn Colvin is no longer Acting Commissioner of Social Security. She has been replaced by Nancy Berryhill. If you are suing the Social Security Administration, you would now sue Berryhill rather than Colvin. If you already have a suit pending against Colvin, you don't have to do anything. This situation is addressed in §25(d) of the Federal Rules of Civil Procedure:
Public Officers; Death or Separation from Office. An action does not abate when a public officer who is a party in an official capacity dies, resigns, or otherwise ceases to hold office while the action is pending. The officer's successor is automatically substituted as a party. Later proceedings should be in the substituted party's name, but any misnomer not affecting the parties’ substantial rights must be disregarded. The court may order substitution at any time, but the absence of such an order does not affect the substitution.