how do i remove/replace the sim in 6s plus

The sim tray is on the right hand edge of the phone about halfway down. You will need to eject the old sim with the sim removal tool that came with the phone (if you don't have this tool you should be able to use an unfolded paper-clip).

If your phone is carrier locked (tied to one particular mobile phone company who subsidised its original price of the phone), then the replacement SIM might need to be from that same company, or you could ask them to release the carrier lock.