To Access SharePoint from the Teams Application

1. Click on the Teams tab in the left side menu.
   
2. Select the team and channel that you wish to open the respective SharePoint Team site for.
3. You can easily click Files for a quick view of the files in the Team:
   
4. In the toolbar at the top of your working screen, click the button to Open in SharePoint
   

5. Note: If you do not see the Open in SharePoint button, click the ellipsis button in your tool bar. From the menu select Open in SharePoint.
   
   Note: You can also bring up the Open in SharePoint option by clicking on the ellipsis button in the Posts section of your team channel.
   

6. This will open the Team site in your web browser.

For more information on SharePoint please see the following articles from Microsoft:

What is a SharePoint team site?

Collaborating with Teams, SharePoint, and OneDrive

Get users for Microsoft Teams