How to send an SMS in iOS 10
So, does the other person have an iPhone or something else? If it's something else, then it will automatically send as an sms. If the messages fails to send, and you try multiple times to send it, it will ask if you want to send as an sms instead of an iMessage (at least I think it will still do that).

If anything else, you could always go into iMessage settings, sign out of your Apple ID and then turn off and on iMessage.