To change the name of a sheet:
Double-click on the tab at the bottom of the sheet you wish to change. (This tab may say, for example, 'Sheet1'.)
A Sheet Name window will appear.
Type the new name in the Name for Sheet box.
Click OK.
The new name will automatically appear in the tab at the bottom of the sheet.
Note: There is another way to bring up the Sheet Name window. Go to the Format drop-down menu, put your cursor over Sheet, and click Rename.