Here are a few things you can do if your phone’s screen brightness is changing on its own:

1. Restart the phone. This is a simple and effective way to reset your phone.
2. Check auto-brightness. Some phones have an auto-brightness setting that adjusts the brightness level depending on the environment. If this setting is on, it could be the cause.
3. Check settings. Check your settings to see if you have any other brightness options that could be impacting your display.
4. Update software. Make sure you have the latest software for your phone. This can help keep your phone running smoothly and troubleshoot any issues.