How would you improve it?

“How would you improve it?” 
vs.
“What would you improve?”

The latter is satisfied by “I don’t like this aspect and that one”. It invites destructive feedback.

Asking “how”, on the other hand, asks for constructive feedback. You can’t just respond with what you don’t like. You’re invited to share how you would make it better.

And then, a conversation can start about whether that would actually be better. And possibly lead to a third idea that’s even better than that.