The guarantee is claimed by contacting support. That makes preparation worth a few minutes.
Your order number, the email address used for the order, the purchase date and the pack you bought. A short, polite statement that you are requesting a refund under the 60-day guarantee. Photos of the bottles if there is any problem with them.
The page does not list steps, a form or a contact method beyond the support route, so check the refund policy page too.
Send your request in writing well before day 60 and keep a copy. Note the date and time, and ask for confirmation of receipt and the next steps. If the policy says anything about returning bottles or return shipping, follow it exactly. Keep a screenshot of the policy as it read when you ordered.