To activate a different sheet in a workbook you need to click its sheet tab. Also you can press Ctrl-Page Up to activate the previous sheet, and Ctrl-Page Down to activate the next sheet.

But if your workbook contains many sheets, and not all of the sheet tabs fit on the screen, you may find it laborious to scroll through the tabs or use the keyboard to activate a distant sheet. This situation is worsened if you need to keep switching sheets back and forth.

The easiest way to jump sheets is to  display a pop-up list of sheet names by right-clicking the sheet tabs navigation bar (the one with arrows) to the left of the sheet tabs that are located at the bottom of the worksheet window. Select a sheet from the list, and you're there in a flash.