In order to upgrade your License Server 9.0 to version 9.1 please find the following guide below:
Uninstall the eCognition License Server version 9.0
Return the licenses in the uninstaller wizard
Click on the "Return" licenses button and select all licenses
Click on "Return" again and finish the uninstall wizard.
After the License Server 9.0 is uninstalled you can proceed with the installation of the License Server 9.1