In SSIS I want to set variable value from an external configuration file. let's say I have [User::FullPath] variable and I want to set it value from an external config file so I can change my path dynamically.
You have not specified what uses you need for this "variable", and this important for the answer.
Anyway, as far as I know, it depends on your deployment model on SSIS.
Integration Services supports two deployment models, the project deployment model and the legacy package deployment model. The project deployment model enables you to deploy your projects to the Integration Services server.
Deploy Integration Services (SSIS) Projects and Packages
For Package deployment model
Legacy Package Deployment (SSIS)
See "Package Configurations".
For Project deployment model
Setup Environment Variables in SQL Server Integration Services