Woocommerce – Change the template?

I found the solution here WooCommerce: Change template for single product page

But, it doesn’t actually explain how to do it.. or i’m just not understanding enough.
I did give it a go and placed that text in the “single-product.php”, changing where it says dirname to/products.php (my template file name), but nothing changed.

When I go to the product page, I want to be able to use that drop down box that should appear on the right of the page to change the default template as the style on the product page is incorrect.

Can someone please dumb this down for me? I’m pulling my hair out as that’s the only thing left to do on my website, I can then make it live.

Thanks!