Step 1:

Open up CMD as an Administrator and run the following command;

Slmgr.exe /upk

Step 2:

First you need to get GVLK Key to proceed;

Server EditionProduct GVLK
Windows Server 2019 DatacenterWMDGN-G9PQG-XVVXX-R3X43-63DFG
Windows Server 2019 StandardN69G4-B89J2-4G8F4-WWYCC-J464C
Windows Server 2019 EssentialsWVDHN-86M7X-466P6-VHXV7-YY726
Windows Server 2019 Azure CoreFDNH6-VW9RW-BXPJ7-4XTYG-239TB
Windows Server 2019 Datacenter Semi-Annual Channel (v.1809)6NMRW-2C8FM-D24W7-TQWMY-CWH2D
Windows Server 2019 Standard Semi-Annual Channel (v.1809)N2KJX-J94YW-TQVFB-DG9YT-724CC
Windows Server 2019 ARM64GRFBW-QNDC4-6QBHG-CCK3B-2PR88
Windows Server 2016 Standard Semi-Annual Channel (v.1803)PTXN8-JFHJM-4WC78-MPCBR-9W4KR
Windows Server 2016 Datacenter Semi-Annual Channel (v.1803)2HXDN-KRXHB-GPYC7-YCKFJ-7FVDG
Windows Server 2016 Datacenter Semi-Annual Channel (v.1709)6Y6KB-N82V8-D8CQV-23MJW-BWTG6
Windows Server 2016 Standard Semi-Annual Channel (v.1709)DPCNP-XQFKJ-BJF7R-FRC8D-GF6G4
Windows Server 2016 ARM64K9FYF-G6NCK-73M32-XMVPY-F9DRR
Windows Server 2016 DatacenterCB7KF-BWN84-R7R2Y-793K2-8XDDG
Windows Server 2016 StandardWC2BQ-8NRM3-FDDYY-2BFGV-KHKQY
Windows Server 2016 EssentialsJCKRF-N37P4-C2D82-9YXRT-4M63B
Windows Server 2016 Cloud StorageQN4C6-GBJD2-FB422-GHWJK-GJG2R
Windows Server 2016 Azure CoreVP34G-4NPPG-79JTQ-864T4-R3MQX
GVLK Keys

Step 3:

Open up CMD as an Administrator and run the following command; Use GVLK Key from the above table according to the edition.

Dism.exe /Online /Set-Edition:ServerStandard /ProductKey:XXX-XXX-XXX-XXX-XXX /AcceptEula
Dism.exe

It will take some time to complete then it will go for restart;

License Activation

Step 4:

Once Windows Server 2019 is back open up CMD and execute the following command; This time use your original key which you have purchased.

slmgr.exe /ipk ABCD-EFGH-IJKL-MNOP-QRST
Windows Server 2019 License Activation

Next you have to run the following one;

slmgr.exe /ato
Windows Server 2019 License Activation

Step 5:

Once you are done, you can verify by using the following command;

Dism.exe /Online /Get-CurrentEdition
Dism.exe Online

You are done !

Windows Server 2019