How to change default enfold theme to flat business theme.

Hey Amol,
This can be done two ways, the first and best is to reset the site with WordPress Reset it will clear all content, styles, images and be like you started from day one.
Then activate Enfold, and install the demo. Remember, you will lose everything.

The second option is if you need to keep your posts & pages & images, you will need to export your posts and pages though WordPress > Settings > tools > export, and then use a plugin like downML – Download Media Library and save your media library to your desktop, which you will upload again one by one later.
Then continue with the steps above.

If you must try to save all of your content, then please make a backup with the tools your webhost has to use, just in case you make a error and need to try again.

You can try to just install the new demo over your current demo, but your menus will double up with items from both demos, and you may still lose some posts, maybe not. Either way you will need to spend some time cleaning up the site afterwards, it’s not the best way to do it

Best regards,
Mike