How would you approach an issue where a customer says their drink is wrong?

I will go to the customer’s table and politely ask them to explain the problem and will understand what drink they ordered and what they got. If I will found that the drinks got mixed up, I will apologize for it and make the new drink right away.

If there is some other issue with their drink such as level of sweetness or something else then I will ask them to share their taste and will upgrade the same drink accordingly.