Change Add to Cart to Read More

loops.php

add_filter( 'woocommerce_loop_add_to_cart_link', 'mw_change_add_to_cart_loop' );
function mw_change_add_to_cart_loop( $product ) {
    global $product; // this may not be necessary as it should have pulled the object in already
    return '<a class="button" href="' . esc_url( $product->get_permalink( $product->id ) ) . '">DETALHES</a>';
}