Hover over the column whose width you wish to adjust.  The column will be outlined in blue.

To adjust the width, click the circles on either side of the column and drag it until it reaches your desired width.

Percentages may appear to guide you.

Another method to adjust column widths is to click on Column Settings and edit the width through the popup window under the Style tab.

To reset columns to their original widths, hover over the Edit Column button and click Reset Column Width.

When finished editing the page, click Done, then Publish in the top right corner.