# ...change the default margins?

The easiest way to change the margins is with the geometry package. You’ll need to include the package in the preamble and you set the margins by including them as options. Typing this line is in the preamble

\usepackage[margins=7cm]{geometry}

will set a margin of 7 centimeters around the entire document. To specify
each of the 4 margins individually we need to include more specific options.
Include this in the preamble

\usepackage[left=1cm, right=3cm, top=3cm, bottom=4in]{geometry}

to set a left margin of 1 centimeter, right margin of 3 centimeters, top margin of 3 centimeters, and bottom margin of 4 inches. By adjusting the value of left, right, top, and bottom you can set the margins to whatever you want. I've included a tex file where you can uncomment these commands in the preamble and see the effect on the margins: ChangeMargins (.tex)