it’s Very Easy ,Follow Some Easy Step
=> Launch the Messages app on your iPhone or iPad running iOS 8 or higher
=> Tap on the thread you’d like to leave in order to open it.
=> Tap on Details in the top right
=> Tap on Leave this Conversation — you’ll need to scroll down a ways.
Remember that you can only leave group messages if all participants are using iMessage and running iOS 8 or higher. Since Apple doesn’t have access to block or remove you from carrier messages, iMessage threads are the only thing they can remove you from on their end. The Leave This Conversation option will be grayed out like you see above if someone in the thread is running an older version of iOS. If the bubbles are green, you won’t be able to remove yourself since those are regular text messages.