If you mean in your web page then simply change the fonts defined in the CSS file attached to your pages.
If you mean for all pages displayed in your browser then simply specify the font you want in the browser's user CSS file and make sure the option is turned on so that the browser will lokk at that file as well as the one supplied with the page.
If you want, you can set up your own style sheet for a website. You'd only do this if it's a site you visit often, though. In most browsers, you can install a style sheet that will apply to a particular site you visit. The process is different for each browser, though. What browser do you use?