Learn how to delete messages and conversations on your HTC One. On the Messages screen, do any of the following:
Delete a message: Open the conversation thread with a contact, tap the message, and then tap Delete message.
To delete multiple messages within a conversation, tap Menu icon > Delete messages > Delete by selection, and then select the messages to delete.
Delete a conversation: Press and hold a contact (or phone number), and then tap Delete. To delete multiple conversations, on the Messages screen, tap Menu icon > Delete threads, and then select the conversations to delete.
In the Messages settings, tap General > Delete old messages to auto delete old messages.
How do I protect a message from being deleted?
You can lock a message so that it will not be deleted even if you delete the other messages in the conversation.
On the Messages screen, tap a contact (or phone number) to display the exchange of messages with that contact.
Tap the message that you want to lock.
Tap Lock message on the options menu. A lock icon is displayed.