[Tracker] How to update a field efficiently

["Chen, Zhenqiang" <zhenqiang chen intel com>]

Hi, 
 
I have a photo which has GPS info. After query from the GPS service, I get the "city" corresponding to the 
GPS location. Now I want to writeback the "city" to "mlo:city". 
 
How to update it efficiently by UPDATE rather than DELETE ... INSERT...?  What's the sparql like?  
 
Thanks!
-Zhenqiang