USCIS has updated Form N-400 Application for Naturalization. As of December 23, 2016, USCIS will ONLY accept the new version. Also, as of December 22, 2016, USCIS will require the new fees to be paid for N-400 and all other USCIS forms that have been subject to the fee increase.
You can find the new USCIS Form N-400 here: https://www.uscis.gov/n-400
You can find the USCIS fee increase list here: https://www.uscis.gov/forms/our-fees
Whenever you send a form to USCIS, make sure to check the required form version on the day that you postmark the application. Likewise, whenever you submit fee to USCIS, make sure to the check the required fee on the day that you postmark the fee. Periodically USCIS changes form versions and sometimes eliminates earlier versions from being accepted. If you submit a form that is outdated or an incorrect fee, USCIS will almost always return the application and fee to you. Generally, you can refile the form and fee with the corrected version or fee. If you must file a form by a specific deadline, however, USCIS may not forgive a late form or fee that is the result of you having originally submitted the incorrect form version or fee.