Translation:How do I get to the post office?
Could it be both? There's a verb やる, to do, which in it's -te form looks like やって. Combine it with どう and you could get something meaning how (do you do X) or "in what way." My guess though is that even if that was part of the word some time ago, by now どうやって has become a word of its own.
Is there someone who really knows the answer?
I think it should. I put "How do I go to the post office", but "How do I get to the post office" sounds more natural to me.
I put "How will you go to the post office?", and I can't figure out why that translation is inaccurate.
I'm pretty sure that should work, it's just kind of an unusual way to translate it
Alwsys assume its the speaker asking about himself in DuoLingo. Unless otherwise specified. "How do I", not "how will you".