Click on the Administration gear in the right corner of the page, select More... from the bottom of the drop down menu to open the Course administration page.
From the Course administration page, click on the Reports tab.
From the Reports page, click Dates.
From the Dates page, you can filter your Activities by type, or Expand all (right side of the page near the top).
From the page that opens, you can update all dates at once.
Don't forget to Save changes before you leave the page.