Open the workbook that will contain the external reference (the destination workbook, also called the formula workbook) and the workbook that contains the data that you want to link to (the source workbook, also called the data workbook).
In the source workbook, select the cell or cells you want to link.
Press Ctrl+C or go to Home > Clipboard > Copy.
Switch to the destination workbook, and then click the worksheet where you want the linked data to be placed.
Select the cell where you want to place the linked data, then go to Home > Clipboard > Paste > Paste Link.
Excel will return the data you copied from the source workbook. If you change it, it will automatically change in the destination workbook when you refresh your browser window.
To use the link in a formula, type = in front of the link, choose a function, type (, and then type ) after the link.