Travis Jordan wrote:
Usually the cover plate assembly screws on. If you broke the bond
between the cover and the assembly that screws into the sprinkler head
then try removing the assembly by turning it counterclockwise. Don't
be a dolt and cause the sprinkler to go off.

http://www.sprinklerreplacement.com/...Residental.pdf

Also this:

http://www.tyco-fire.com/TD_TFP/TFP/TFP442_07_2005.pdf