Once your logged in, you want to make sure that you're working in "back end editor" mode instead of classic. To check, look above your WYSIWIG editor and click the button that says "back-end editor" If you do not see that button above, then great - you're already in back-end editor mode.
Next scroll down to the column that you want to modify. Click on the pencil in the middle to modify that column.
Finally, click on the tab that says responsive options to see how the element can be laid out.