I am new to DynamoDB and trying to learn, while going through the docs I got a doubt whether DynamoDB is version agnostic or not also I want to know wether it is backwards compatible or not? In addition I also want to know how to download local DynamoDB of different versions if possible and how can I change the version of DynamoDB in the webservice of aws ?
dynamoDB is a cloud hosted datastore. It it not a piece of software that you can download or install. As far as I know, there is no version, and you can't/don't have to select any version anywhere.
At most you can use some docker containers that mimics the behavior of dynamoDB to some extend. They are useful for testing and dev purposes, but keep in mind they are just mocks.