Go to prometric.com, search for your test (USMLE or NBOME, not COMLEX because that isn't how COMLEX is listed), click on change date, type in your confirmation number, search for the site you want, search for the date you want, click on your new date, hit submit.
Go to prometric.com, search for your test (USMLE or NBOME, not COMLEX because that isn't how COMLEX is listed), click on change date, type in your confirmation number, search for the site you want, search for the date you want, click on your new date, hit submit.