Hello Vasanth,
Yes, this is possible - the same way you do it in the client side event (I guess you are using the setValue method documented here):
http://help.infragistics.com/Help/NetAdvantage/NET/2008.2/CLR3.5/html/WebGrid_cell_Object_CSOM.html
I guess the problem here is to locate the cell instance. This is easy and can be achieved by using th eigtbl_getCellById global grid method documented here:
http://help.infragistics.com/Help/NetAdvantage/NET/2008.2/CLR3.5/html/WebGrid_Utility_Functions_CSOM.html
If you do not know the ID of the cell, you can enumerate the Rows collection of the grid on the client and get the cell by index as well.
You can also update the cell on the server-side with AJAX - this way there will be not visible refresh no screen (but requires roundtrip to server, so it might be slower).
Finally, if possible - please write directly in the grid forum for grid related technical issues - chances are you will get faster/better replies there
http://forums.infragistics.com/forums/61.aspx