How to Flash Your Nexus S (or Any Other Android Device) with a New ROM
 

When you bought your phone it was cutting edge, had the latest version of Android, and made your heart sing. A year or more later, it doesn’t get the new updates and the performance is a little sluggish. Breathe new life into your phone by flashing it with a new custom ROM.

http://www.howtogeek.com/162516/how-...ith-a-new-rom/