function updateContactMethod() {
  var value = this.options[this.selectedIndex].value;
  if(value == '1') {
    document.getElementById("contact_method").innerHTML = '+375';
  } else {
    document.getElementById("contact_method").innerHTML = '+7';
  }
}
<select id="contactSelect">
    <option value="0">Россия</option>
    <option value="1">Беларусь</option>
</select>

<p><span id="cart-price-1" class="cart-price">от 200 RUB набор</span></p>
<p><span id="cart-price-2" class="cart-price">от 400 RUB шт.</span></p>

<p><span id="cart-price-14" class="cart-price">от 2000 RUB/шт.</span></p>