The company has started rolling out the One UI 4.0 update based on Android 13 to Galaxy S20. A Reddit user has posted that he has received a major update on his AT&T Galaxy S20. The update brings the latest One UI 4.0 based on Android 12 to the device. The build number of the new update is G981USQU2EULH.
You can manually check for the update from Settings > Software update >Download updates manually. If there is no update showing on your device, then wait for a few days for the update to come to your device as it is being released phase-wise manner or you can manually flash the full firmware using Odin Flash Tool.
