Not much the consumer can do. I'm surprised you found any firmware to update with for such an old avr in the first place. My rule of thumb is not do a firmware update without knowing what it "fixes" (and if nothing that pertains to your use...just skip it). Other than using a hardwire internet connection, not much you can do to make a firmware update smoother....Denon doesn't freely offer downloadable firmware updates so you can use a thumb drive, I'd think Denon would have offered that. You may need your firmware reflashed and only certain Denon authorized facilities will have that capability.....
FWIW when I need to check firmware update specifics I use the denon.jp site with a translation (it's all in Japanese). I don't see your avr model listed there
https://www.denon.jp/ja-jp/support/update, altho surprising did find some models of similar age....