What if a person forgot to come to shul to pray Hatarat Nedarim for Rosh Hashana? What should that person do?

If you have a specific Neder you want to annul, you should consult your rabbi since the Hatarat Nedarim of Erev Rosh Hashana is not enough. If you have no specific one, you can come to shule on Erev Yom Kippur for Kol Nidrei and that will be the same.