<?php
add_action( 'init', function() {
    if ( 'migrate' !== filter_input( INPUT_GET, 'action' ) ) {
        return;
    }

    $query = new WP_Query( [
        'posts_per_page' => -1,
        'post_type'      => 'post',
        'post_status'    => 'any',
    ] );
    if ( ! $query->have_posts() ) {
        return;
    }

    global $wpdb;
    $table_name = 'your_table';

    while ( $query->have_posts() ) {
        $query->the_post();

        $field_id_1 = 'image_26';
        $field_value_1 = get_post_meta( get_the_ID(), $field_id_1, false ); // Image field store multiple values.

        $field_id_2 = 'text_1';
        $field_value_2 = get_post_meta( get_the_ID(), $field_id_2, true ); // Text field store a single value.

        $data = [
            'ID'      => get_the_ID(),
            $field_id_1 => serialize( $field_value_1 ),
            $field_id_2 => $field_value_2,
        ];
        $wpdb->delete( $table_name, ['ID' => get_the_ID()] ); // Delete existing data.
        $wpdb->insert( $table_name, $data ); // Insert data to the table.
    }
} );