Select and option is shown by default in HTML if you don't set any selected options for drop down. So if something is set you need to look for words like selected= "selected" in from of an option value and remove that
You would need to look into the liquid code and remove "selected" from an option. Can you share your URL and specific page where you want to achieve this, I might be able to suggest better.
You may want to take help from here
Hope this helps.