I originally filed a 1040EZ, how/what form do I use to file amended for dividend 1099 rcvd late?

Answer 1

You need to complete and mail form 1040X
http://www.irs.gov/pub/irs-pdf/f1040x.pdf

Answer 2

You always use a 1040X to file an amended return, but if you want to prepare a new return so you can lay the original and the corrected one side by side to help in filling out the 1040X, use a 1040A since 1040EZ doesn't allow reporting of dividends.