DSXchange

I have an environment variable for tns service name as for example a_b, but due to some reasons i want to change to a.b. How can i change it so that it can reflect in all the jobs in the project ?

In a single project on a server where multiple projects exist? You'll need to add the Environment Variable to the project via the Administrator and give it the new value there. That value will automatically be set for every job's environment in that project when each job runs.

Is it the value of the environment variable or the name of the environment variable that you wish to change globally?

If it's the latter, then I don't see that you have any option other than to edit an export file.