The second one sounds more natural. I have never heard the first one used in this context.

If you wanted to change the topic of conversation you could say/we say: Changing the subject, or... On a different subject. If the subject had changed midcourse and you don't understand how it happened, you could ask: How did we end up talking about this/talking about ...(mention the name of the topic)? You could also say: How did we end up talking about ....? I hope this helps you. 😀