Click Settings app.
iPhone settings are displayed. Scroll down the page till you see Messages.
Messages Settings are displayed. Scroll down the page till you see Keep Messages.
Click Keep Messages.
Keep Messages settings are displayed where 'Forever' is enabled by default. Here you can click on either 30 days which would mean that the messages would automatically delete 30 days after you receive them or to 1 year which would mean they would automatically delete 1 year after you receive them. So now, if you are a heavy SMS or iMessage user, you won't need to manually delete your messages.