There are a few potential steps you can take to fix Face ID on an iPhone when it keeps saying "move phone higher" or "move phone lower":
Make sure that you are holding the phone at the proper distance from your face. Face ID works best when the phone is held about an arm's length away from your face.
Make sure that there is enough light in the room. Face ID may not work as well in low light conditions, so try increasing the lighting in the room or moving to a well-lit area.
Make sure that your face is not covered by anything. Glasses, hats, scarves, and other objects can block the Face ID sensor and prevent it from working properly.
Restart your iPhone. Sometimes, simply restarting the device can resolve issues with Face ID. To restart your iPhone, press and hold the Sleep/Wake button (also known as the Power button) until the "Slide to Power Off" slider appears. Then, slide the slider to turn off the device. After the device is off, press and hold the Sleep/Wake button again until the Apple logo appears.
Update your iPhone to the latest version of iOS. Outdated software can sometimes cause issues with Face ID. To update your iPhone, go to Settings > General > Software Update and follow the prompts to install the latest version of iOS.
If these steps do not resolve the issue, you may need to reset the Face ID settings on your iPhone. To do this, go to Settings > Face ID & Passcode, enter your passcode, and then tap "Reset Face ID." This will delete all of the stored Face ID data on your device and allow you to set up Face ID again.