I want to change <b> to an another font ... but where and ho

Dont do that!

By replacing <b> with <font..> you'll lose all semantic information that comes with <b>. Its like replacing "bold, more important" by "yum, funky fonts and colors".

You should change the font using CSS. For example:

div#text b {
    font-family: Helvetica, Arial, "sans serif";
}