Manage Inbox Allows you to erase, lock, or unlock your Inbox messages. To get there, go to a message that is locked. Left soft key and OPEN the message, then right soft key and select Manage Messages. Choose Manage Inbox to lock or unlock that specific message.
Page 43/44 of your user manual. https://www.lg.com/us/mobile-phones/pdf/Final%20enV3%20UG%20English.pdf