The worksheets in a workbook can be altered in several ways. The name of a particular worksheet can be altered. New sheets can be added. A current sheet can be duplicated or removed. The sheets can be reordered. Other sheets operations can alter the colour of the tabs or change the "protection" status of a worksheet to allow cells to be locked or hidden.
The names of sheets can be changed. You can do this by editing the tab directly. Clicking twice on a tab will highlight the text and then standard editing operations will alter the text contents.
You can insert a new empty sheet after the current sheet through the Sheet menu entry in the Insert menu or through the context menu which appears when you click the right mouse button on a tab.
Instead of an empty sheet, you can add a copy of the current worksheet to the workbook after the current sheet by selecting the Duplicate this Sheet menu entry from the context menu.
You can remove the current sheet using the Remove this Sheet menu entry from the context menu.
You can re-order worksheets by clicking and holding the left mouse button on a tab, then dragging that tab to the left or to the right. Little arrows will appear showing the location where the sheet will be placed. Releasing the mouse button will then move the sheet to this new position.
Many of these operations can be performed at once from the Manage Sheets dialog which can be opened through the Manage Sheets... menu entry in either the Sheet submenu in the Edit menu or in the sheet tab's context menu.