There are two ways:
- Tap the "three dot" button then choose Settings / Other / and choose Clear default reader apps. After you do this and assuming that more than one app for a format is available, CC will ask you again for which app when you read a book.
- Tap on a book to open book details. Tap the "three dot" button and choose "Open book with ...". You will be presented with a list of available reader apps. Choose the one you wish to use. Check the "Set as default for ..." check box before choosing the app if you wish to make your choice the default.