After you create a schedule, you may want to change its organization and structure by grouping columns. You can create several layers of headings and subheadings to provide more detail in your schedule.
Be sure the cursor appears as an arrowhead as you select the headings.
Or Shift + Click to select multiple headers.
Headers panel
Group, or right-click the selected headings, and click Group Headers.
A new heading row displays above the grouped column headings.

Text in the new heading row is centered.
Tips
Headers panel
Ungroup, or right-click in the heading cell, and click Ungroup Headers.