Your iPhone will scan your photos for faces, and create albums for people but it doesn't always pick the best key photo

To change it, open Photos and in the Album tab scroll down to People & Places. Click People and choose a person

Press the Select button mid-right, then scroll until you find the photo you want

Make sure you only select one photo, otherwise it won't work

Hit the Share icon bottom left, then scroll the bottom row of icons all the way to the left

Find the icon with a tick on it called Make Key Photo and press it

You'll now see your selected photo as the main image for that person