upgrade cash rewards to cash rewards plus

hi! I have had the original cash rewards card for awhile now and I wanted to upgrade to the cash rewards plus card but I am a little confused. When I fill out the application it just says cash rewards and not plus. Do I submit an application for another cash rewards card and get the plus if I'm approved or ask for a limit increase on the current cash rewards card and that will become the plus if I'm approved? Sorry for confusion and if this has been asked before