If a customer want to be a seller how to do that?

I’ve solved it. My work around is to create a shortcode button, and a form. Put it on a page of your preference where only logged in user can see the page.

On the hit of submit button, update the role to seller, and update the meta key: ‘dokan_enable_selling’ to yes on usermeta table.

add_shortcode ('tombol_jadi_seller','jadi_seller');

function jadi_seller() {
    if (current_user_can('contributor')) {
        echo '
        <form method="post">
            <input type="submit"  name="submit" value="Saya Ingin Jadi Seller" class="button">
        </form>';
    }
}

add_action ('wp_loaded', 'my_init');
function my_init() {
    if (isset($_POST['submit'])){
        if (current_user_can('contributor')) {
            $id = get_current_user_id();
            $user = new WP_User( $id );
            $user->set_role( 'seller' );
            update_user_meta( $id, 'dokan_enable_selling', 'yes');
            wp_redirect (get_site_url().'/dashboard');
            exit;
         }
    }
}