Open Symantec Encryption Desktop application
From Symantec Encryption Desktop window
From Encryption Disk or Partition
From Unlock Disk pop-up window
Once Decryption is completed and Windows 10 Installer is available follow below procedure or continue to step 6.
Once Decryption is completed
Now you can update Windows 10 and start the encryption once Windows Update is completed.
As often happens, please try not to get confused between SEE and SED. You've linked a SEE article but provided pictures of SED here.
The article for upgrading Windows on a machine protected by SED can be found below:
HOWTO128174 How to upgrade computers encrypted with Symantec Encryption Desktop to a Windows 10 release Last Updated December 17, 2019 https://www.symantec.com/docs/HOWTO128174
It is important because the software versions are different, and you'll want to first upgrade to SED 10.4.2 MP4 before running the Windows 1909 upgrade.
Thankfully, as both products now use similar underlying technology, both support the "WINSETUPAUTOMATION" flag, so you can upgrade without decrypting:
TECH255563 Updating computers encrypted with Symantec Encryption Desktop automatically to a Windows 10 feature update Last Updated July 31, 2019 https://www.symantec.com/docs/TECH255563