Dicen que no se puede cambiar... pues, ¡cómo no! si se llevan la tajada más grande del pastel.
They say it can't change... well, of course! if they take the biggest piece of the cake.
Captions 3-4, Andrés Manuel López Obrador - Publicidad de TV - Part 2Play Caption
The setup line here, Dicen que no se puede cambiar, translates to:
"They say that things can't change."
Then we have the simple phrase ¡cómo no!, which is translated as "of course!" Taking it word by word, cómo (with an accent over the first ó) means "how," and no means "no" or "not." But "how not!" is not quite as straightforward as the simple "of course!" in our translation. Context can be most helpful here. So, ask just about any soccer (fútbol) fan if they'll be watching the World Cup finals on Sunday and the reply in Spanish is the same: ¡Cómo no! / ("Of course!")
Next comes, si se llevan la tajada más grande del pastel, or "if they take the biggest piece of the cake." Note that the phrase la tajada más grande del pastel can also be phrased el trozo más grande de la tarta.
You see, both pastel and tarta mean "cake." At the same time, both trozo and tajada mean "slice" or "piece." And your choices don't end there: Another way to say "a piece" or "a bit" is un pedazo, but that's not necessarily culinary. It's often used in the sense of "to fall to pieces" (caerse a pedazos). Meanwhile, una porción is commonly "a portion" but it can also mean "a slice" as in, una porción de pizza.
Got all that? Don't worry if you don't find it's "a piece of cake," which, incidentally, is expressed in Spanish as no está chupado or, no es pan comido.