When you delete a text message on your iPhone, it's removed from your history permanently, unless you're using a version prior to 3.1. If you're using an iPhone that hasn't been updated, there's a good chance that any text messages you delete are still on your iPhone, and you can search and find them if you try. Unfortunately, there is no way to remove these deleted messages unless you upgrade your iPhone to the latest version.
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Plug your iPhone into your computer and use iTunes to download any updates. If your iPhone is still using a version prior to 3.1, text messages won't be permanently removed when you delete them.
Click the "Messages" icon.
Swipe to the left to reveal the Delete button, which will appear to be a red circle.
Tap the Delete button to remove the conversation from your iPhone.