Hi,

Please do following steps here to do that:

1) Install & active plugin Customizer Reset & plugin Customizer Export/Import

2) Go to Appearance > Customize > click to button "Reset" at the top to reset all your customizer data to default.

3) Import demo style ( .dat file ) you want to import to your site - by do following ONLY Way: 2 in this guide - Way 2: Import Only customizer data

Note: 

- If you want to switch to another demo style, you just need to do again step 2) and 3)

- After switch to another demo, if you have the issues about menu items appears over screen - you need to go to Appearance > Menus > create/select a menu > scroll down to bottom > and check to "Primary Menu"

Best Regards,

PenciDesign