How do I fix an iphone 7 plus that had an error on update and is stuck on the apple logo downloading?

I think we're past the stage of being able to recover any data. I hope you had a backup.

See if tips in this post help (it is a bit old but should still apply)

hhttps://discussions.apple.com/message/33175214#message33175214

If none of the above work, try:

https://appletoolbox.com/iphone-stuck-in-recovery-mode-here-are-4-steps-to-fix-it/

If you can't update or restore your iPhone, iPad, or iPod touch - You can put your iOS device into recovery mode, then restore it using iTunes.

https://support.apple.com/HT201263