UPDATE A
SET
    A.USR_TramitadorTesis = B.idUsuario
    FROM asurnet.dbo.SINIESTROS A
         INNER JOIN(
    SELECT C.IDTSS as idUsuario,A.SIN_Id
    FROM asurnet.dbo.SINIESTROS A
    INNER JOIN (
        SELECT * FROM tssseg0001.dbo.IDTraspasos
        WHERE TableID = 'Usuarios'
    ) C ON A.USR_Tramitador = C.IdTras
) B ON A.SIN_Id = B.SIN_Id