The C20 is a very old, end of support device. Support for it ended more than 3 years ago.
You will not be able to upgrade it from TC5 without a valid release key, which you could only get from Cisco while the device was still supportable.
Unless you have some very good contacts within your local Cisco Account Team, or within the Cisco TelePresence BU, getting a release key now will be next to impossible.
It would be highly recommended to forget about the very old device and replace it with something current and supportable.
Wayne
Please remember to mark helpful responses and to set your question as answered if appropriate.
