Device to cloud

I have the Kindle app on my iPad, and since I'm quite new to the Kindle community I've been downloading books to the device instead of the cloud. Can I move all those books from device to cloud. I don't want to delete them, I just don't want them cluttering up the library screen. I did the tap-and-hold thing but I'm afraid that if I remove from the device I'll end up deleting them. Thanks for any help you can give.