To make a specific worksheet active for editing, you can click on its sheet tab. Excel lets you know what sheet is active by displaying the sheet name in boldface and making its tab appear on top of the others. The active sheet tab is a different color from the inactive sheet tabs.
On the bottom left of the worksheet you will find the Sheet Tab scroll buttons – First sheet, Previous sheet, Next sheet, and Last sheet. They are followed by the tabs for the worksheets in your workbook and the Insert Worksheet button or tab, depending on the version of Excel you are using. If your workbook contains too many sheets for all their tabs to be displayed at the bottom of the worksheet area, use the Sheet Tab scroll buttons to bring new tabs into view (so that you can then click them to activate them).
If you prefer, right click on one of the Sheet Tab scroll buttons and a list of the sheets will appear. If there are more sheets than can be displayed, More Sheets …. appear at the bottom of the list. When you click on a sheet name, you will be moved to that sheet and it will be the active sheet.
You also can use the Ctrl+Page Down and Ctrl+Page Up keyboard shortcuts to activate the next and previous sheet, respectively, in your workbook.