How to export/import all blog posts from old website to new?

First of all please export your previous posts from your old blog via WP Dashboard => Tools => Export:

http://prntscr.com/12imcpi - download first XML file.

and export media in the similar way and download second XML file: http://prntscr.com/12ime9z

Then go to your new website in WP Dashboard => Tools => Import => Install WordPress importerhttp://prntscr.com/12imfa2

Run importer and upload first XML file => click "Upload and Import" with the following settings: http://prntscr.com/12imgqk

Pay your attention to the option:  Download and import file attachments. It would be activated.

Then upload and export the second XML file with media.

Then copy all media from the /wp-content/uploads of old website to the /wp-content/uploads of new (stage) website.

As result, you will receive your blog posts with media inside in your new blog: https://prntscr.com/12imio1