how do I transfer epub files from my laptop to my ipad

If they don't digital-rights-management on them (if they do then check the site that you got them from for which apps you can use for them on your iPad) then you can add them to your computer's iTunes library via the File > Add To Library menu option - if using a Mac with OS X Mavericks on it then add them to its iBooks app via File > Add To Library.

Then connect your iPad to your computer and select it in your computer's iTunes, and use the Books tab to select and sync the epubs to your iPad's iBooks app