Convert Add to Cart Button (not for all products, just a few) into a link to an external webpage

Site URL: https://carlotona.com/family

Hi there, 

I would like to convert my 'Add To Cart' (not for all products, just a few) to a button that links the customer to an external website where he/she can buy the product. Not all of the products on the given URL can be purchased on my website, hence this action is required. What I have done so far is simply adding a different section below adding images with the products linking them to the external website. 

Is there anyone that can help? I've never coded myself, hence my question: Is this something easy to do by adding a code or is it likely to be more tricky and affect my other products that can be purchased on my site? 

I hope you understand what my intention is! 

Thank you in advance!

Please see the following.

Let us know how it goes.