Learn Spanish (Latin America): Ella volvió a apostar en la ruleta ayer.

In Spanish, the phrase volver a followed by an infinitive verb is the most common way to say you do something again. Instead of saying 'she bet again' (apostó otra vez), Spanish usually says literally 'she returned to bet' (volvió a apostar).

Since this lesson focuses on verbs of change, is volvió acting as a verb of change here?

No, but it is very important not to confuse the two structures! When volver is used with a and an action (volvió a apostar), it just means doing that action again. To use it as a verb of change meaning 'to become', you must make it reflexive (se volvió) and follow it with an adjective, like se volvió rica (she became rich).

Why is it en la ruleta for 'on roulette' instead of sobre la ruleta?

When you bet money 'on' a casino game like roulette or poker, you use the preposition en. You will also see a used when betting 'on' a specific outcome, person, or sports team (like apostar al caballo ganador, to bet on the winning horse). The literal translation of 'on' (sobre) is generally not used for betting.

Why is volvió in the preterite tense here rather than the imperfect?