There should be a clickable "unvote" above the comment if you wish to do so.

Regarding anything else about how Hacker News works, I found this quite useful:

https://github.com/minimaxir/hacker-news-undocumented/blob/m...

Yes - after you upvote, a link that says "unvote" will appear above the comment. E.g. the following text currently appears to me above your comment:

Scottopherson 5 minutes ago | unvote | parent | flag | favorite | on: The value of downvoting, or, how Hacker News gets ...

Don't feel bad that you didn't notice - I was reading HN for a long time before I realised that un-upvoting was possible.

Accidental upvotes used to be a problem, but that's been fixed for a while now. You should see the word "unvote" appear https://i.imgur.com/jaNanv9.png

Downvoting is enabled when your karma reaches a threshold (500?). You can undo votes in either direction via the “unvote” or “undown” links that appear after voting. I think this is only possible for a short period of time after your original vote.

I guess there are a few more karma points to go for you. Not sure what the exact threshold is.

The UX is subpar on mobile. Too many tiny links too close to each other.