Also , when i use a PATCH data action in above link , i do get a success messsage in data action side , but the value is not updating on SF side .Please suggest .
I cannot help too much with questions on how to use the salesforce API/how to get it working. This section of the forum provides help if you have problems with the Genesys Cloud API or our SDKs. For general questions about salesforce, I would recommend the Coffee House category for you post.
However, we do have a few example salesforce data actions in our GenesysCloudDevOps GitHub org. I don't know if you have any Terraform experience, but these are all remote modules that can be deployed in your org very easily. If not, no worries, the exported data action for each exists in the repo as a json file.
Here are the salesforce data actions that perform a POST:
One piece of advice that I can give is that you are best off to master how to use a rest API with a tool like postman prior to implementing a data action. Are you able to use a different tool to practice inserting and updating salesforce records?