If you want to Convert PST to MBOX but you don't know How its possible then I want to Recommend you a third party tool Regain PST to MBOX Converter Software that has capability to Convert PST to MBOX file format with all mailbox data items.
This tool is also able to convert both .ANSI and .Unicode PST files into mbox format without any hassle.
Users can also try this software free demo version and where they can save 30 emails per folder at free of cost.