Go to the theme.liquid file in editor and replace these lines with your existing title and meta description code.

<!---- Title and description ---->

<title>
{% if product %}
Buy {{ product.title }} Online India – {{ shop.name }} {% else %}
{{ page_title }} {% if current_tags %} | tagged "{{ current_tags | join: ', ' }}"{% endif %}{% if current_page != 1 %} | Page {{ current_page }}{% endif %}{% unless page_title contains shop.name %} | {{ shop.name }}{% endunless %}
{% endif %}
</title>

{% if product %}
<meta name="description" content="Buy {{ product.title }} at {{ shop.name }}. Enjoy best deals, free shipping, offers and much more on it. Shop Now!">
{% else %}
{% if page_description %}
<meta name="description" content="{{ page_description | escape }}">
{% endif %}
{% endif %}

Another example of meta tag description

{% if page_description %}
   
    {% if product %}
        {% if collection.title == "For Her" %}
            <meta name="description" content="Buy Fitstrong {{ product.title }} online at {{ product.price | money }} from Fitstrong supplements. You can choose & purchase a desired supplement to attain your fitness goals. Shop now!">
        {% elsif collection.title == 'For Him' %}
            <meta name="description" content="Buy Fitstrong {{ product.title }} online at {{ product.price | money }} from Fitstrong supplements. You can select & buy a desired supplement to attain your fitness goals. Shop now!">
        {% else %}
            <meta name="description" content="{{ page_description | escape }}">
        {% endif %}
    {% endif %}
  
{% endif %}