Do I need to report both W2's if I received another W2 in which the employer has now added missing information to 12d: DD and $ amount?

You can combine these two W-2s (i.e., if you already entered the first W-2, then only add to it the new entries that appear on on the 2nd W-2).  Do not type in "Corrected" in Box 14.

Or if you have not entered any W-2 information yet, then just enter this 2nd W-2.

DO keep both W-2s with your tax records.

how do I make  changes to my tax form

Just go back into the W-2 like you did the first time and make the corrections that are on the updated version. If the 2nd version has "Corrected" checked on it, then put a check mark in the "corrected" box in the form on your computer.