Hi Bojzon,
1. Enter the number '1' in any unused cell.
2. Copy the cell
3. Select the range that you want to modify
4. Choose Edit|Paste Special, then 'Values' and 'Multiply'
5. Click OK.
If you need to do this often, you could set up a macro to automate the process, and attach it to a button or keyboard shortcut.
Cheers