What about iPhone 6plus

iMessage is or is not working. If it is working as you say, then what is the problem? If it is not working, then you have a problem. See if this helps: If you can't send or receive messages on your iPhone, iPad, or iPod touch - Apple Support

If your question is about updating your iPhone model 6, you can't do that. See why in this compatibility chart:

See the link I suggested.