You can easily edit an existing Report by clicking on it's title in your Reports Dashboard.
As soon as you open your Report, it will show you the columns and filters you set the last time you saved it.
In order to edit the columns, click the Field Select button on the top right of your screen and check or uncheck the button of the Fields you want to be displayed or not to be displayed any longer.
You can edit the filters you applied on the left hand panel of your Report by again going to the Field on your Report and then changing or deleting the applied filter.
As soon as you're satisfied, click Save Report again and your new layout is saved!