Devices with a 32 GB hard drive or older devices with full hard drives, might need additional storage space to complete the upgrade. If you don’t have sufficient space, try removing files or apps you no longer need or use Disk Cleanup to free up space.
During the upgrade, you might be asked to free up space on your device or attach an external drive with sufficient space to continue with the upgrade. If you attach an external drive, make sure to keep it in a safe place after the upgrade in case you need it for recover options.