How I can change back to the blue message?

Thanks for dropping by the Apple Support Communities, ravikg01,

After reading your post, it sounds like you were previously seeing blue message bubbles when using the Messages app, and now see green. We would be happy to provide you with some information on Messages and what each of those colors mean!

Blue bubbles indicate you are sending an iMessage. iMessages are sent over any internet connection (Wi-Fi or cellular) from your Apple device to another person who is also using an Apple device and iMessage. These messages can be sent using the phone number from your iPhone, or your Apple ID.

Green bubbles indicate you are sending an SMS/Text message. SMS/Text messages are sent exclusively over your wireless carrier's cellular network to anyone with a mobile phone number. These messages can only be sent using the phone number from your iPhone.

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

If you were previously seeing blue bubbles for your contacts who also use iMessage and no longer do, we want to start by checking a few things:

1. Make sure iMessage is enabled on your iPhone: Set up Messages on iPhone - Apple Support
2. Make sure you are connected to the internet, whether it is Wi-Fi: Connect to Wi-Fi on your iPhone, iPad, or iPod touch - Apple Support or your carrier's network: Set up cellular service on iPhone - Apple Support
3. Make sure the person you are messaging is using an Apple device, and has iMessage enabled on that device.

If all of the above conditions are met and you still are not seeing blue bubbles on your iMessage messages, please don't hesitate to respond back.

Best!