Messages - How do I change who is sending the message?

If in a group chat

1. Tap the avatars at the top, and the avatars will expand

2. Tap the avatar you want to change to and you are now sending incoming messages as that user.

If not in a group chat

Simply just tap the speech bubble in the message bar to switch between incoming and outgoing.

Please ensure that you have tapped the speech bubble in the Message bar to switch between incoming and outgoing messages.