If you're having sync issues in Teams, a banner appears at the top of the app containing a Refresh link. Click the link to restart your app; you should be fully synced once everything is up and running again.
Your other option is to manually restart the app by right-clicking the icon in your dock (Mac) or taskbar (Windows) and selecting Quit. After you quit, just click the app icon to open it again.
Our mobile apps automatically sync every time you open or use them. You can also pull down to refresh your messages.