postman collection variables not changing in header settings code example

Example 1: how to define variables in api

-Use pm.globals to define a global variable:

pm.globals.set("variable_key", "variable_value");

-Use pm.collectionVariables to
define a collection variable:

pm.collectionVariables.set("variable_key", "variable_value");

-Use pm.environment to define an environment
variable (in the currently selected environment):

pm.environment.set("variable_key", "variable_value");

-You can use unset to remove a variable:
pm.environment.unset("variable_key");

-Defining local variables
Local variables are temporary values you set
in your request scripts using the following syntax:

pm.variables.set("variable_key", "variable_value");

Example 2: environment variable api

Environment Variable , a variable that accessible
only when the environment is active 

Usually used for an app with multiple environment ,
we can use env variable to store variable with same name
and different value to store environment specific data 

for example : 

xxxNamed app has 3 environments with different
URL and Crendentials 

But all endpoints are exactly the same no matter what
environment you work on 

so we can create 3 environment called 
QA1 , QA2 , QA3 and run same set of request 
by selecting different environment.

Only one environment can be active at a time.