Update Column With Sequence Number Every Time

hello friends

i want to update one column from my table regularly on sequence number i.e. 0,1,2,3,4,5

i created procedure but it is not working as per my output

declare @IndexIDGen int
declare @ID int
set @ID = 0
update temp_test set indexid = NULL
declare IndexIDGen cursor for select indexid from temp_test
open IndexIDGen
FETCH Next from IndexIDGen into @IndexIDGen
while @@fetch_status = 0
begin
update temp_test set indexid = @ID where indexid is null

set @ID = @ID + 1
print @id
fetch next from IndexIDGen into @IndexIDGen
end

close IndexIDGen
deallocate IndexIDGen


where i am going in wrong direction ??????

T.I.A