I migrated from android to iphone, whenever I receive a new sms it creates a new thread and doesn't update the existing migrated one

Hi amokhtar10,

I understand that you're seeing some odd behavior when receiving messages from contacts with which you've had prior conversations that have been migrated from another device. In such a situation, you may need to delete the existing conversations as detailed below.

Delete the conversation* that has the issue and start a new conversation:

1. Open Messages and look at your list of conversations.
2. Swipe from right to left on the conversation you want to delete. Then tap Delete.
3. Open Messages, tap , and enter the phone number or email address of your contact or contacts.
4. Write a new message and tap send.

*When you delete a conversation, you won’t be able to recover it. If you want to save parts of conversations or their attachments, you can take screenshots. You can also touch and hold an attachment or message bubble, then tap More > Save.

If you can‘t send or receive messages on your iPhone, iPad, or iPod touch
https://support.apple.com/en-us/HT204065

Regards!