Formal:
* Shift the topic
* Alter the discourse
* Divert the conversation
* Transition to a new subject
Informal:
* Switch gears
* Move on
* Change the tune
* Go off on a tangent (this implies a sudden shift, perhaps unintentional)
More nuanced:
* Introduce a new topic
* Bring up something else
* Segue into something different
Politer options:
* Would you mind if we discussed something else?
* Perhaps we could talk about something else?
* I'd like to change the subject for a moment.
The best synonym will depend on the specific situation and your relationship with the person you're speaking to.