Please do not attempt to do it if the site's content is essential to you. Each theme has its Builder and bundled plugins, and there is no way to automatically make it work with another builder and plugins. This is valid for any theme. The only exception is when the old and the new theme are built with Identical builders and plugins, but this is extremely rare. 

1. If you want to keep the content of your existing site until the new one is completed, put up the staging site, install the new theme on a fresh WordPress installation, and then transfer the content to the new theme. Note that there is no automatic transfer. You will need to transfer it manually, page by page. 

2. In case you don't need the content of your existing site, you need to install the WordPress Reset Plugin and reset (delete the content) the database to guarantee the best performance of the theme.

* Please be aware that all your current databases will be deleted once you use this plugin. 

WP Reset – Most Advanced WordPress Reset Tool Plugin – This plugin is the fastest way to eliminate content. However, it will remove all content from your database and activate the default theme. It does not remove plugins but will deactivate them and remove all pages, posts, menus, sliders, widget data, theme options, etc. It removes all the content from the existing site. Click Here To Download.

3. If you're resetting the database due to a corrupted installation, deactivate all the plugins before deleting the database. After the database reset, delete all of the plugins and the theme. Next, you can follow the installation guide steps for set up the theme and import the demo content.