Is there any difference in meaning betweeen these two?
Is one a valid grammar construct and the other not?
We don't have any context, so this might be wide of the mark, but I'm going to assume it's something like...
So long as you are cleanly and modestly clothed, you can dress how you want/will.
Having narrowed things down a bit, my first observation is that many if not most speakers would actually say however rather than how in such a sentence. But I really can't offer any support for that observation, nor suggest any good reason why it should be so.
What I will say is that how you will has (to me at least) significant overtones of being archaic, literary, and/or 'perfunctorily dismissive'.
That's to say, how[ever] you want suggests an indulgent speaker willingly granting his audience the freedom to choose, possibly even with the implication that the exercising of that freedom may be desirable from his point of view too. Whereas how you will suggests that the speaker either doesn't care about such details at all, or does care, but knows he's unable to enforce his will.