Open a command prompt as an administrator and depending on the version, run either of these:

Dism /online /Set-Edition:ServerDatacenter /AcceptEula /ProductKey:12345-67890-12345-67890-12345

OR

Dism /online /Set-Edition:ServerStandard /AcceptEula /ProductKey:12345-67890-12345-67890-12345

This is useful if you are seeing a ‘cannot upgrade from this edition’ error message.