Thank you for your question and your post.
The Nokia belle upgrade will not be available for the Nokia C6-00. The reason being the operating system on Nokia C6-00 is Series 60 5th edition and the Anna and then Belle upgrades where only available on Symbian ^3 operating systems.
Hope this answers your questions.
If you wish to check the latest version of software for your device then can either update via Nokia Suite or over the phone. I have a link below with instructions on how do to this if you are unsure:
Symbian^3 and Symbian S60 3rd edition are different operating systems. Symbian^3 is the latest and the most advanced version of the Symbian operating system. Current software version of Symbian^3 is Belle. Belle and Anna can only be updated to the devices that have Symbian^3 as the original software.
Please refer to the link provided by Raoul_Duke regarding to the software updates for C6-00.