When logged in, go to "My Folder" in the upper right part of this page. It is recommended that you open a new window (by right-clicking on the link and choosing "open in new window" in the popup-menu), so you can keep these instructions visible.

Click on the entry of the paper:

In the "states" menu, click on "Retract":

Click on "edit" in the tab above the green bar:

Now you can edit the description (title or abstract) of your paper and/or upload a new version using the upload control:

Don't forget to click on "Save" after you have selected the file.

Remember to re-publish your paper by choosing "submit" in the "states" dropdown menu. Otherwise, it is not visible!