datatable.Select(string.Format("[lineNo]='{0}' and [Position]>='{1}' ", lineNo, Position)).ToList().ForEach(r => r["Linetext"] ="Sample Text" );

hi praveen Donga
u place a sqldata source control and gridview control on to webpage .then u go to grid view control properties we have a property datasource id=sqldata source control. then configure the sqldatasource control by giving table. and then u use the update statement by clicking on update tab  u use the following syntax
<update tablename set  column name=value,columnname=value where columnname=value>