Do you want to put new frequencies in memory to replace what's already there? If that's what you want, I think you have to follow the instructions for entering frequencies, thereby removing what's already there and simultaneously putting what you specifically want in that memory location.
If you just want all memories cleared, you might need to do a full reset. I'm guessing, based on ham equipment. I've never owned an RCI (or any CB equipment). A full reset "should" restore the radio to fully just-out-of-the-box condition with nothing stored in memory.