How do I change my dark red hair( looks almost like a dark brown with red) to a medium dark brown with caramel highlights?

If the dark red is a darker level than the brown (as it seems from your description), you have to use color remover first (ColorFix, Color Oops, Color Zap, etc. Don't use Effasol!). After that you can re-dye it with the lighter medium dark brown and highlight over that with bleach.