Each team has a corresponding SharePoint site. If you edit or add a file to a channel Files tab, use the Sync button at the top of the tab to update the SharePoint. Likewise, if you make any changes to the team's SharePoint, use the Sync button to update the team.
You can also sync updates from your cloud storage via Files on the left side of the app. Go to the cloud storage of your choice, then select Sync at the top of the page.