Hi @kiakia ,
If you are looking for the latest version of the C API and the associated release notes they can be found on this page: https://solace.com/downloads/
If you would like to view older versions of the API and their associated release notes then that could be done using your organizations login credentials at https://products.solace.com/ .
Please let me know if you need any more clarifications or have further questions!
Cheers,
MV