Transact SQL :: How To Delete Row With Condition

create table target
(Taid int, studenid int, std int,div int,year int ,ActivityId int)
insert into target values(1,1,1,2015,1),
(1,1,1,2015,2),(1,1,1,2015,3),(1,1,2,2015,1)
,(1,1,2,2015,4) ,(1,1,2,2015,3);

create table activity(ActivityId int,Activity nvarchar)
insert into activity values (1,'sport')
,(2,'swim')
,(3,'read')
,(4,'dace');

I want to edit(update,delete,insert) target table activity from front end for that I took grid view and that grid all activity come of single student studenid(unique key for each student) that you see design below like this 

that activity present in target table that come as check in check and that activity not select student from activity activity table that come unchecked

I want to update above information means  unchecked alredy check data should delete and new check data add , already check data update

I have use merge statement but problem is that at single time only one row execute that is check because  data come from data gridview.