Often, you don't need a special word to change or introduce the topic.
When you do, you can use 'regarding', but it's quite formal. A more informal way to introduce a topic that might be unexpected is "you know".
"You know that dance tonight - will you be able to come?"
"About the dance tonight - will you be able to come?"
or "We were talking about", if it's something that had been spoken about previously and you are re-introducing it.