Learn Spanish (Latin America): Ustedes están mirando cómo cambia la superficie. - You all are looking at how the surface changes.

Estar + gerund is used for an action happening right at this exact moment ('are looking at'). If the sentence used the cumulative periphrasis vienen mirando, it would mean you all 'have been looking at' it continuously or repeatedly over a period of time leading up to now.

Why is the word order 'cómo cambia la superficie' instead of 'cómo la superficie cambia'?

In Spanish, when a question word like cómo (how) introduces a clause, the verb usually comes right after it, before the subject. So, the English 'how the surface changes' naturally translates to cómo cambia la superficie.

Why does 'cómo' have an accent mark if there are no question marks in the sentence?

Question words like cómo, qué, and dónde keep their written accent marks in indirect questions. The phrase 'how the surface changes' is an embedded question inside the statement, so cómo keeps its accent to distinguish it from the word como (which means 'like' or 'as').

Could I use 'viendo' instead of 'mirando' here?

You could, but it changes the nuance. Mirar means 'to look at' or 'to watch' intentionally, while ver generally means 'to see' passively. Because the people are actively observing the surface to see how it changes, mirando is the most natural fit.