Search "epub reader" on Google or in the app store of your device and you'll find dozens of options to choose from. Here though are some good, free EPUB reading apps we'd recommend starting with:
iBooks: http://bit.ly/ZrnAVa
Bluefire: https://itunes.apple.com/it/app/bluefire-reader/id394275498?mt=8
Aldiko: http://bit.ly/AuxTp0
Freda: http://bit.ly/X25Q1B