Use this guide to replace the SIM card.
Gently insert a plastic opening tool between the rear panel and the inner rear frame near the rear-facing camera.
Use your plastic opening tool to pry up the rear case around the perimeter of the Nexus S.
Remove the rear panel from the Nexus S.
Using the edge of a plastic opening tool, lift the battery from the end closest to the headphone jack.
Remove the battery from the Nexus S.
Use your thumb to slide the SIM card downwards from the SIM card slot.
Remove the SIM card.