Please help, I would like to modify the 2 x AP PMP450 to work as PTP 450. What should I do, please advise

Sorry I meant SM and not AP. I would like to use the two SM as a PTP Link

Two PMP450 SM's cannot participate in a point to point role. SM's can only be used as a client. You'll need a PMP450 AP and a PMP450 SM to create a point to point link. You will see slightly better performance however if you use the actual PTP450 radios instead, or even better yet, the PTP650.