Hi @Jack ,
You can check the Exchange version using Get-ExchangeServer | select Name,AdminDisplayversion
Schema version indicates that Active directory preparation has been done with the Exchange server media.
For this upgrade, you can ignore the existing values and follow the below steps,
1.Ensure that all Exchange 2013 system requirements are met
https://learn.microsoft.com/en-us/exchange/exchange-2013-system-requirements-exchange-2013-help
2.Install the pre-requisites - Especially .NET Framework
https://learn.microsoft.com/en-us/exchange/exchange-2013-prerequisites-exchange-2013-help
3.Prepare the Active Directory - You can also verify the same values once the AD preparation has been done with CU23 setup
https://learn.microsoft.com/en-us/exchange/prepare-active-directory-and-domains-exchange-2013-help
4.Install the Update using GUI or unattended mode
https://learn.microsoft.com/en-us/exchange/upgrade-exchange-2013-to-the-latest-cumulative-update-or-service-pack-exchange-2013-help
If the above suggestion helps, please click on "Accept Answer" and upvote it