While it is possible to add "Select an option" or similar text to your variant dropdowns on product pages using this tutorial provided by Shopify, we recommend against making this update.
First, this update requires some more advanced changes to the code, so it is very easy to make a mistake that could break your entire theme.
Second, this change tends to interfere with a variety of third party apps and can cause your entire store to stop working properly.
Finally, it's worth noting that, from a user experience standpoint, there's typically little to be gained from implementing this option. Most Internet users are adept at knowing that they can click a dropdown menu for more options and understand that they should do that before adding an item to the cart.
In addition, making this change can actually make your store more confusing because users may not understand which image in your image gallery matches the selected option.
Also, don't forget that it's also always possible for users to edit their own carts to make any changes to items with incorrect variants that are added to their carts by accident.
Please note that if you attempt to implement this, neither Shopify or Out of the Sandbox support can assist with issues that arise because this is considered an advanced customization.
If you do want to modify your theme to include this feature, you will need to update the following files:
- app.js.liquid in the Assets folder
- product-template.liquid in the Sections folder
- product-details-template.liquid in the Sections folder (only if you use the product.details page)
- Any additional product-details-template files you created
- product-form.liquid in the Snippets folder (plus any additional product form files you may
If you are not comfortable implementing this change, you can hire a developer to help you.