Tools - Marking Cells - Unmark Selected Cells

Ribbon bar. Select the Tools tab. Click Marked Cells and from the menu, select Unmark Selected Cells...

Classic menus. From the Tools - Marking Cells submenu, select Unmark Selected Cells (or click the toolbar button located on the Tools toolbar)...

...to remove the selected cells from the set of marked cells in the spreadsheet (i.e., the Marked Cells spreadsheet layout will be removed from the selected cells). Any previously marked cells that are not selected will remain marked. See also: Format - Overview, Spreadsheet Layouts - Overview.