Norma, are the floors in good shape that they only need to have a new coat of poly applied to them? If so, then clean them real good or any dirt/stains/dust will become a permanent fixture. After that, clear off the floor and apply the poly down. You should wait a full day before walking on it again.
You need to prep the floor first. You would do this by doing a screen sanding with a very light abrasive pad. Once the original finish dries the new finish will not adhere very well. Add to that even though they are in good shape. Any cleaning or spraying of chemicals in the air may have left a film that you cannot see. If this is not removed the new finish will fail. Much like painting, if you do not rough up the finish the new paint will not stick and eventually begin to flake off. Same happens with floor coatings. Had the finish just been put down the new finish coat would be able to bite into it on a chemical level. But because it has dried and has had some use. It must be prepped properly to get a good job done.
Screen sanding is very easy to do. Its done with a buffing machine that can be rented with a foam pad on bottom. Then a abrasive screen material is placed between it and the floor. You simply buff the floor with this removing very little finish, Just enough to dull the older top coat. Just be sure you use the finest grit screen you can get assuming the floor is in as good of shape as you say. You do not need to spend lots of time on this. Go over the floor once or twice fairly quickly just enough to dull the surface and remove any stuck in grit from the original finish and your good to go.
Once done you vacuum and then damp wipe and clean floor off to remove any left over dust from the sanding then refinish with what ever top coat you want. Just be sure however that what ever you use can be put over the existing top coat you just sanded.
Follow the directions on the use of the coating you choose. Be sure to allow enough time prior to walking on it and putting any furniture on it as well. Even if it feels dry it takes several days for it to completely harden deep enough to resist scratches and marking.