today, we have a license adobe indesign server multi instance creative cloud
but we need install indesign server cs6 and we don't have the serial number for the installation .
how can I downgrade the license please ?
Talk to the reseller who sold it to you. I'm not sure there's anything to do (except paying for a full license) since CC is a subscription license while CS6 is a perpetual one. But your reseller is the only one who can really help you.