You can't and please do not attempt to do it,  if the content of the site is of any importance to you. 

1. If you want to keep the content of your existing site until the new one is completed, then put up the staging site, install the new theme on a new 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 once you use this plugin, all your current database will be deleted. 

WP Reset Plugin – This plugin is the fastest way to get rid of content. However, it will remove all content from your database and leave the default theme activated. It does not remove plugins, but will deactivate them and remove all pages, posts, menus, sliders, widget data, theme options, etc. Basically it removes all the content from the existing site. Click Here To Download.