Do not dump it. Do not add anything now. If this was an extract batch and you added the correct amount of water your OG is not off - no matter what your hydrometer told you. It is near impossible to thoroughly mix the dense, sugary wort from the kettle with the less dense, pure top-off water. You may think you mixed the snot out of it, but you didn't. Your real OG is probably very close to the estimate.
Now, if this was an all-grain batch...or you added too much water...well, that's a different story. Still I'd probably leave it alone for now.