Text message

The following articles will explain why.

Leave a group text

In the Messages app, you can leave a group text at any time, as long as there are three other people on the thread. Here's how:

1. Go to the group text that you want to leave.
2. Tap the top of the conversation. 
3. Tap , then tap Leave this Conversation.

To leave a group text, all participants need to be using iMessage. If you don't see the option to leave, it might mean one or more of the users isn't using a device with iMessage. You can't remove yourself from these group texts, but you can mute the conversation.

How to leave a group text on your iPhone, iPad, or iPod touch - Apple Support

How to Leave a Group Chat in Messages for iOS on iPhone and iPad - OSXDaily

Add or remove someone from a group conversation

Anyone in a group iMessage can add or remove someone from the conversation. You can remove a person from a group iMessage that has at least three other people. You can’t add or remove people from group MMS messages or group SMS messages.

Send a group message on your iPhone, iPad, or iPod touch - Apple ...

https://support.apple.com/en-us/HT202724

The OP's question was how to leave a group text, not add or remove someone.