Deleting a message or conversation does not delete it from your contact's phone. You can only delete off of your own phone. Going forward, you can set disappearing messages with this contact where the message disappears after a certain time after the message has been read.
To delete individual alerts or messages
- Open the chat with your contact.
- Long press the message bubble or alert.
- Tap on any other message bubble you want to delete or skip to step 4 for alerts.
- Tap the trash can .
- Confirm yes.
To delete conversations
- Go to the main list of conversations.
- Long press on the conversation.
- Tap on any other conversation you want to delete.
- Tap on the trash can .
- Confirm the delete.