How to change your current K2.Net Server license:

Open K2.Net Service Manager
Expand your server group
Right click on the K2 Server and choose properties
Click on your K2.Net Server (VM2003)
Replace the current license key with the newly provided key
Make sure that Processor Config is set to 1
Click the update button
Restart your K2 Server service.
Open K2.Net Service Manager and check if the license has changed.