You can't do that most of the time.
Originally Posted by Gixxer07
The only time you can do that is if the message has not been "delivered." So if, say, you connected to a WiFi network that is experiencing trouble with Internet you won't see "delivered" for iMessages. But tapping and holding on the message will allow you to send as text instead, which uses the cellular network not internet.
Since iMessage messages "usually" are delivered right away this is why those responding to this thread don't see the send as text option.