My advice is to not do it. There is some risk involved because any kind of oil or soap or floor polish / wax can prevent the urethane from adhering.
if you want to take the risk , I recommend using a disc buffer with a maroon 3M pad and Bonakemi prep. Get in touch with the Bonakemi company
http://www.bona.com/ And their tech people can walk you through the process.You'll basically spritz the floor with the Prep and then abrade it with the buffer and 3M maroon pad. If the pad shows no signs of contamination ( grease or wax build up on it) then you thoroughly vacuum / tack the floor clean and apply a new coat of finish.
This is easier said than done if you aren't an experienced finisher.