I already have a SIM from my old service. What should I do with the new SIM I just received?

You'll need to remove the old SIM card and insert your new Optimum Mobile SIM. Toss the old card out and you are good to go. Depending on your phone, the SIM card slot could be on the top, side, or under the battery cover. If you cannot locate it, check your phone's manual or the manufacturer's website.