If your existing Windows 10 PC is running the current version of Windows 10 and meets the minimum hardware specifications to run Windows 11, it will be able to upgrade.
Specific timing for upgrade rollout will vary by device and certain features will require specific hardware—see https://www.microsoft.com/windows/windows-11-specifications for more information.
To see if your PC is eligible to upgrade, you can download and run the PC Health Check app. Or, after the upgrade rollout begins, you can check if it's available for your device by going to Settings > Update & Security > Check for updates.