First of all, you should avoid staring directly at her hips or making inappropriate comments or actions. This will make her feel very uncomfortable and may lead to the deterioration of your relationship.
Secondly, you can try to divert your attention, such as turning your eyes away, or changing the topic to avoid embarrassment.
Finally, if you feel you have to say something, you can try to make your language and behavior look mature and respectful. For example, "I'm sorry, maybe I accidentally saw something I shouldn't have seen just now." I hope you don't feel uncomfortable. I will pay attention. "
In a word, staying calm and respectful is the key to deal with this situation. Avoid inappropriate behaviors or remarks and protect the dignity and self-esteem of girls.