To change your background in a 1-on-1 or group video call:
1. Tap the video call screen.
2. At the bottom of the screen, tap the effects icon > Backgrounds.
3. Select a background photo.
You can set a photo saved on your device as your background by tapping "+". Choose a photo, edit it any way you like, then tap Done.
To remove a background, tap the effects icon > Backgrounds, then select the no background icon at the top left.
Also, you can use the iPhone's Portrait mode (the mode that keeps you in focus and blurs the background) in video calls. On the video call screen, tap and hold the video call icon, tap System Video Effects, and turn Portrait ON.
Note: This feature is available on devices from 2018 or later running iOS 15 or later. See the following Apple Support page to check if your device supports Portrait Mode.