Open the Messages app
Tap the conversation containing the message
Press and hold the specific iMessage
Tap Undo Send (or Unsend)
Confirm by tapping Undo Send again, if prompted
If Undo Send is not available, resend the message is not possible with iMessage; delete it instead
To delete, press and hold the message and tap Delete
On iMessage, you can also delete the whole conversation: open Messages, swipe left on the conversation, tap Delete