Greetings, I want to update a historic process instance variable, as a part of my business requirement. Is it even possible, if yes then which API should I use for the same?
No, the API' in Activiti don't allow that: once it's in history it is set in stone.
If you need it though, you will need to add your own Command implementation. In that Command, you'll have access to the lower level entityManager which would allow you to update historical data.