。
She tuned the TV to channel 6.
Literal
She [topic-は] TV [object-を] channel-6 [to-に] tuned.
合わせる ('to match, fit, adjust, set') is a high-utility verb that pairs with many domains: 時計を合わせる ('set the clock'), 顔を合わせる ('meet, come face to face'), 力を合わせる ('join forces'). For TV/radio it captures the act of dialing in to a specific frequency or channel. The に on ６チャンネル marks the target of the matching — 'set X to Y' uses Y-に in Japanese where English uses 'to.' This whole TV-tuning scene presupposes the era of dial-tuned analog televisions; modern remote-controlled sets would more often pair with 押す ('press') or 切り替える ('switch').