Without knowing the layout of your spreadsheet and whether 'every' cell has to be multiplied, here is a way to multiply selected cells or ranges of cells:
1. Enter 1000 into a cell outside the range of cells you wish to change.
2. Select the cell containing 1000 and Copy it
3. Select cells with values to be changed - you can select a range of cells
4. Right click and select Paste Special
5. Select the Multiply radio button ('All' will be selected in the top section by default)
6. Click OK
All the selected cells should now show original values x 1000.