trigger to update a top 1 row

hii all

i have two database server's A and B respectively, A have database DB and in that a table called tbl_A like   A.DB.tbl_A  and B have database DB_B and in that a table called tbl_B like   A.DB_B.tbl_B

now i have required to update A.DB.tbl_A as it founds  row in A.DB_B.tbl_B i.e. top 1

how can i do this?? help plz