For example we have cluster in AWS ECS with 3 services:
And we want to change evn variables in api-staging service.
To do it we should create a new revision of task definition with updated env variables and then tell the service to use new task definition.
So, to create a new revision of task definition we should go to list of all revisions, select the last one and create a new revision.
To change the container evn variables we should go to container definitions section and edit our container: 
Set new values to env variables and then update container and save new revision of Task Definition:
When we have a new revision we should update our api-staging service and set the latest revision. Also don't forget to set force new deployment checkbox 
Top comments (1)
When you done to update the task definition, you need to create/replace a new service