With a flat plastic tool (metal may cause damage to the mechanism underneath) it's easy to detach a key from an iMac keyboard. Just be gentle.
This video is part of our guide to replacing keys on Mac keyboards:
http://www.macworld.co.uk/mac/news/?n...
And if you want to see how NOT to remove a Mac key, take a look here:
• How NOT to remove an iMac keyboard key