How do you switch from sms message back to iMessage in a single chat ?

You cannot manually switch. It is done automatically based on whether or not both you and the recipient have an iOS device with iMessage activated, and have an internet connection, be it cell data or wifi.

If something changes and one or more of those things is not true, then iMessage defaults to SMS. Once all is true again, you should be able to send iMessages again.

How did you switch to text message exactly?

If you have internet connection, and your recipients have an internet connection, and an iOS device with iMessage activated, messages should be sent as a blue iMessage.