What is the term for the type of answer "You should not do X. You should do Y instead. Here is how to do Y." to the question "How do you do X?"?

That answer or activity would be an alternative or alternate. It might also be called a counter proposal.

From the responding individual's point of view, you may have an XY problem. Wikipedia says:

The XY problem is a communication problem encountered in help desk and similar situations in which the real issue, X, of the person asking for help is obscured, because instead of asking directly about issue X, they ask how to solve a secondary issue, Y, which they believe will allow them to resolve issue X. However, resolving issue Y often does not resolve issue X, or is a poor way to resolve it, and the obscuring of the real issue and the introduction of the potentially strange secondary issue can lead to the person trying to help having unnecessary difficulties in communication and offering poor solutions.

In How To Ask Questions The Smart Way, under "Questions Not To Ask," Eric S. Raymond writes:

Q: How can I use X to do Y?

A: If what you want is to do Y, you should ask that question without pre-supposing the use of a method that may not be appropriate. Questions of this form often indicate a person who is not merely ignorant about X, but confused about what problem Y they are solving and too fixated on the details of their particular situation. It is generally best to ignore such people until they define their problem better.

I found this about "Frame Challenge" in my favourites in my browser, so "Frame Challenge" may well be what I am thinking of:

https://rpg.meta.stackexchange.com/questions/6842/whats-a-frame-challenge

However, I would still be interested in hearing if there are answers which are more correct.

The user "doppelgreener" mentions in their answer (https://rpg.meta.stackexchange.com/a/6843) that "Frame Challenge" is Stack Exchange jargon, and not recognised elsewhere. Joel Harmon's answer to another question gives some other terms which may be applicable outside of Stack Exchange: https://rpg.meta.stackexchange.com/a/7579

I am unsure if the fact that these questions are in Role-Playing Games Meta Stack Exchange, and not English Stack Exchange, should imply that the answers are only applicable within the context of Role-Playing Games. Because of this, I will not accept my answer as the answer, unless others can confirm that the answers are applicable to general English.