If you already filed taxes and need to amend them do you have to reenter all of your W-2's?

No. You will probably find the 1040-X, (the form used to amend a return), fairly easy. Actually, you only have to input the things that changed...pretty much just on the identified line - like interest income - as filed, the new amount, the change (up or down) - and the resulting totals. True, if your change is from a new or corrected W-2, that may need to be submitted too.