If you want to add a header or footer to all sheets, select every sheet by right-clicking one of the sheet tabs at the bottom of the Excel screen and clicking "Select All Sheets" in the pop-up menu.

Open an Excel file, and decide carefully the worksheets on which you want to apply the common changes. Press Ctrl, and then use the mouse to select the sheets you want to group.

You can sum cells across multiple sheets in Excel. This article presents steps for Adding single-cell values or Adding values in the range of cells.