Magento can be configured to display a notice when your store is in demo mode so that your customers know that their orders will not be processed. When you are ready to “go live,” simply remove the message.
In order to enable/disable the Demo Store Notice, you need to log into your Magento administration area and follow the instructions below:
For Magento 1.9.x and older:
Go to the System tab and select Configuration. Select the Design section, click HTML Head and you will see a Yes/No drop-down menu for Display Demo Store Notice.
Enable/Disable the demo store notice and click Save Config to save the change.
Magento 2.x and newer:
Go to Content and under Design, choose Configuration. Select the store view that you want to configure and click Edit. Under Other Settings, expand the HTML Head section. On the bottom of the page you can set the Display Demo Store Notice to your preference (Yes/No).
Select Save Configuration to save your changes.