Carry out the following steps to edit an existing Home page to add a system notification message using Lightning App Builder:

1. Click on the Setup gear icon, as shown in the following screenshot:

2. Click the Setup option, as shown in the following screenshot:

3. Type app builder in the Quick Find search box, as shown in the following screenshot:

4. Select the Lightning App Builder option.
5. Click the Edit link for the Home_Page_Default customized Home page, as shown in the following screenshot:

6. In the resulting Lightning App Builder page, drag and drop the Rich Text component from the standard Lightning Components pane to the top of the Home page canvas, as shown in the following screenshot:

7. Click on the newly added Rich Text component.
8. Enter Please be aware of routine Salesforce CRM maintenance on 4th July! into the Rich Text properties section.
9. Choose the text font, size, and color using the text formatter in the right-hand section of the Lightning App Builder, as shown in the following screenshot:

10. Click on Save, as shown in the following screenshot:

11. Finally, click on Back.