Currently I am developing in a fresh dev org with no namespace created. I want to push this code to another dev org with namespace already created. How I can push the code with namespace included( Both apex and lightning ). Do I need to change manually or any other ways available?
You would push it just like normal. Changeset (not possible if orgs are not connected), IDE, ANT, etc.
Once it is in the org it will take on the namespace.
Are you running into any issues? If so please be more specific and you will get a more specific answer
It all comes down to the basics, pushing code or metadata is no different than creating it manually.