How do I create a new report similar to an existing report?

I just need to tweak a parameter or two...

If you want to tweak an existing report definition, the best way to do this is to Save as new report definition. In order to do this, click on the definition you want to modify in the Report Definitions screen to open the Setup tab. You can then make the tweak that you need which causes the button to appear:

You then will save the report definition with a new name. If you want to go back to the original report definition, you can either press 'Reset Parameters' or just exit and reopen the report definition. 

and it will appear under 'No job run yet' at the top