Finally i have got the answer.
Bellow are the step.
1) Get user current user id.
<?php $user_id = get_current_user_id(); ?>
2)set new user forum role you wish to change
<?php $new_role_forum_role="bbp_participant"; ?>
3)fire function.
<?php bbp_set_user_role( $user_id, $new_role_forum_role );?>
now check your user in back end and see forum user role.
Full snippet code
<?php
$user_id = get_current_user_id();
$new_role_forum_role=”bbp_participant”;
bbp_set_user_role( $user_id, $new_role_forum_role );
?>
