It's not an adaptation, but 君たちはどう生きるか is the name of a book for young adults that the Japanese audience might be familiar with; the reference will be lost on an English-speaking audience, draining it of some of its meaning and nostalgia. Imagine a film called "The Little Prince" or something...but of course, The Little Prince isn't the same book, with the same themes, as 君たちはどう生きるか. And even if there were a book which the reference mapped onto well, it still feels like referencing a different title in the translation would be egregious.
In that sense, I'm somewhat fond of "The Boy and the Heron", though I acknowledge it isn't ideal and, as someone who prefers more literal translations, wouldn't necessarily be the route I'd take. Without injecting any specific (and thus wrong) reference, it evokes the nostalgic feeling of English childhood fables and aphorisms - and it lends it their feeling of moral weight, too, just as the Japanese title does. It's a good and evocative title.
The literal translation everyone's using, "How Do You Live?", also isn't ideal. There's a couple of grammatical mismatches between the two languages; whereas the "you" in English can be both singular and plural, the Japanese is explicitly plural; and Japanese doesn't explicitly differentiate present and future tense, and here the meaning is actually likely more future tense. But the big problem is that, to my ears, it sounds deeply accusatory in English, along the lines of "How do you live with yourself?" - and given Miyazaki's views and temperament that wouldn't have surprised me! But in truth the intended Japanese meaning is more inquisitive and philosophical. The original title, in the original novel, is from a book inside of the book, left as a question to the reader. In prose I might translate it as, "How will you, readers, choose to live your lives?", but there's countless variations I've considered; none of them are quite right, and to be honest none quite have the original's punch of a good title!
(disclaimer: I'm a reasonably advanced student of the Japanese language but not a native speaker. Apologies if I've gotten anything wrong.)