While we all continue to wait for an edit button, Twitter is currently focused on testing the ability to downvote tweets, and it's going to let more of us try it.
The experimental feature was introduced last July to a small group of iOS users. The downvote button takes the form of a downward-facing arrow icon located to the right of the heart icon. Initially it was a single-line arrow inside a circle, but the latest version is a larger arrow outline as shown in the tweet below.
Testing last year must have returned positive results as the downvote button is being rolled out to more users across Android, iOS, and the web. Twitter Support announced the expansion, saying, "We learned a lot about the types of replies you don't find relevant and we're expanding this test."
As downvoting isn't a public feature, nobody will see them other than those with access to the test and Twitter's development team. If you're lucky enough to be selected, use your new power wisely as your behavior could help swing whether downvoting becomes a core feature of the service. The fact Twitter is rolling out the feature to more users after six months of testing suggests it's more likely to become one than not.