If you want to add custom option then we have total this option:
<select name="sort_by" id="SortBy" aria-describedby="a11y-refresh-page-message" class="collection-sort__input">
<option value="manual">Featured</option>
<option value="best-selling" selected="selected">Best selling</option>
<option value="title-ascending">Alphabetically, A-Z</option>
<option value="title-descending">Alphabetically, Z-A</option>
<option value="price-ascending">Price, low to high</option>
<option value="price-descending">Price, high to low</option>
<option value="created-ascending">Date, old to new</option>
<option value="created-descending">Date, new to old</option>
</select>
Remove below code and add this above code:
<select name="sort_by" id="SortBy" aria-describedby="a11y-refresh-page-message" class="collection-sort__input">
{% for option in collection.sort_options %}
<option value="{{ option.value }}"{% if sort_by == option.value %} selected="selected"{% endif %}>{{ option.name }}</option>
{% endfor %}
</select>
In this you can change text of this option as you want but cant change value