How can I switch a message from text to message?

If you mean changing a message from SMS/MMS to iMessage, it'll be done automatically when possible. You can't send iMessages (blue bubble, end-to-end encryption) to a non-Apple device, or if you have it disabled. To see if you've enabled iMessage, go to Settings > Messages > iMessage and ensure it's toggled on.

What is the difference between iMessage and SMS/MMS? – Apple Support

If you can't send or receive messages on your iPhone, iPad or iPod touch – Apple Support