To undo a Like, Retweet, or Reply that you submitted to Twitter from Slack, you will need to do on Twitter’s website. This process only takes a few moments.

In Slack, navigate to the tweet that you engaged with and look for the header message (i.e., @channel New tweet from @YourCompany). 

Selecting the word “tweet” (as seen in the screenshot below) will take you to that post on Twitter where you can easily unlike the tweet, undo a retweet, or delete your reply.