Hi
U need to specify which item qty need to update,if u update latest entry,then u can't update old values any more,so better to use where condition for update qty value.
If u want to update lastly added value,then u have set id column in that table and make as identity "yes" so it automatic increment id value once insert value in that table,u no need to insert id value,then when u update use following query
Update [Trail] set qty='"+ textbox.text.trim +"' where ID= (select max(ID) from [Trail])
select max(ID) from [Trail]
this query return lastly added row.
Regards
Aravind