GL-B1300 swap LAN and WAN port

The firmware already permits a WAN to LAN switch, but I want to assign the middle LAN port as the new WAN.
Is this possible? Can I get a step-by-step process on how to do it?

A new WAN to replace the old one? Or you want two WAN connection?

You may do this in luci. i.e. you can assign any port to the WAN zone. But I do not have a step-by-step guide.