Go to
http://www.kobobooks.com/library/purchased.html
This takes you to Kobo's "My Purchased Items" area; a list of all books you have bought from Kobo. Next to most (if not all books) will be either a button "ADOBE DRM EPUB" or "DOWNLOAD EPUB".
Clicking the "ADOBE DRM EPUB" will download an ACSM file, which in turn when you open that file will launch ADE (Adobe Digitai Edition) to download the DRM protected ePub.
Clicking the "DOWNLOAD EPUB" will download an ePub directly; it is possible that this will not download with the epub extension but rather with a ZIP extension. If it does have the ZIP extension just rename it to have the ePub extension.