Go to Settings>icloud, scroll to bottom of screen and tap Delete Account. Then log in using a different ID.
If, however, in doing this, you are asked to enter the password of the "other" ID, then you will have no choice but to contact the previous owner and have them enter the password to allow deleting the account. This is part of the new security features in iOS 7 that prevents thieves from wiping a device and then selling it.