My iPhone 13 screen brightness changes by itself

Hello anilram1961,

Thank you for reaching out to Apple Support Communities.

Sounds like you gave Auto-brightness enabled. These steps can help you adjust that setting:

"Adjust the screen brightness automatically

iPhone adjusts the screen brightness for current light conditions using the built-in ambient light sensor.

1. Go to Settings  > Accessibility.
2. Tap Display & Text Size, then turn on Auto-Brightness."

Additional details can be found here: Adjust the screen brightness and color on iPhone

Hopefully this helps. Let us know if the issue persists.

Cheers!