1. In your slack web app, click the down arrow next to your profile.
  2. In the menu that opens up, select Profile and account. 
  3. In the right pane, click the menu icon under the profile photo. 
  4. Select Account settings. 
  5. In the Account window, scroll down to the end of the page. 
  6. Click Sign out all other sessions. 
  7. Enter the password to authorize sign out.
  8. Click Sign out all other sessions.