If someone is speaking about a subject that you're not comfortable with can you say "dewa" to change topic?
No. では (or それでは) is like "Okay then, ..." which is primarily used to continue the current topic based on what was said. では is also used to return to the main issue after digressing, like "Okay now, (let's start ...)".
If you want to switch to a totally new topic, you should use ところで (≒"by the way"), それより (≒"before that"), etc.
