Depending on your iMac model, the bluetooth board may be blue. Use your fingertip to lift the Bluetooth board from its right edge, disconnecting it from its socket on the logic board.

Be sure to lift the Bluetooth board from its right edge only. Trying to lift it up from any other side will shear the connector off the Bluetooth board.

Use the flat end of a spudger to pry the Bluetooth antenna connector up off the Bluetooth board.