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

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

Selecting LinkedIn post (as seen in the screenshot below) will take you to that update on LinkedIn where you can easily unlike or delete your comment. 

To un-share an update, you will need to navigate to your posts (on LinkedIn) and delete the update. Select Me from the top navigation and then Posts & Activity from the menu that appears. Look for the update you want to remove, select Delete Post from the ellipse menu and confirm that you want to remove the post.