The Changeling's Change Appearance ability states:
You can’t duplicate the appearance of a creature you’ve never seen, and you revert to your natural form if you die.
You decide what you look like, including your height, weight, facial features, the sound of your voice, coloration, hair length, sex, and any other distinguishing characteristics. You can make yourself appear as a member of another race, though none of your game statistics change. You also can’t appear as a creature of a different size than you, and your basic shape stays the same; if you’re bipedal, you can’t use this trait to become quadrupedal, for instance. Your clothing and other equipment don’t change in appearance, size, or shape to match your new form, requiring you to keep a few extra outfits on hand to make the most compelling disguise possible.
So it's all spelled out in the ability's description. As long as you've seen the person you want to look like then you can most certainly change your appearance to that of that specific person, as long as that person isn't a different size to you has the same basic shape as you.