From your default fonts, I'm assuming you are running Word 2007. If so,
there are multiple ways to get there, but I usually show the list of styles
by clicking on the lower right hand corner of the "Styles" box on the Home
tab. This will show a list of styles. If the TOC ones are not visible, click
on options at the bottom of the list, and "show all" if necessary, but I
doubt it will be since you already are using TOC styles in your document.
Then right click on the TOC style you want to modify, and click um...
modify.
This will take you to the standard style modification screen, where you can
make any changes you like. I would suggest only changing one thing at a time
if you are new at this, but it sounds like you only want to change the font
itself, so no worries.
Also, there is a wonderful review of Word TOC features and functions at:
http://www.shaunakelly.com/word/toc/CreateATOC.html
Have fun.