You can change this text in \themes\woopress\woocommerce\checkout\review-order.php file.
Find this code and replace ‘Place order’ to ‘Proceed’ $order_button_text = apply_filters( 'woocommerce_order_button_text', __( 'Place order', 'woocommerce' ) );
this problem we still didn’t fix. The thing was that inside folder checkout there was no review-order.php file but only 3 files: form-billing.php, from-checkout.php, from-cupon.php. I thought some files are missing and I download from woocommerce other files (including file review-order.php) and I insert them inside folder Checkout. Once I did that my checkout page become really strange.