If you are in Easy Mode, click the Retrieve an Old Sheet button. Or,
If you are in Advanced Mode, click the button that looks like an open folder. Or,
Go to the File1 menu, and select Open. Or,
Use the Ctrl+O keyboard shortcut.
An Open Sheet window will appear, allowing you to select the workbook you wish to open. Here, you can:
Search for a file by selecting folders or directories in the Look in box.
Choose to only see files of one type by selecting a type from the Files of type box. This may be useful if you know the type of the file that you are looking for. To see files of all types, ensure that All Files is selected.
Select a file by clicking it once, then clicking the Open button. You may instead click the file once and hit the Enter key. Double-clicking on a file should also cause it to open.
Your workbook of sheets will be retrieved, and ready for you to modify.