Open up the report you want to customize and click Customize near the top. If you don't see the Customize option then click Copy Report and click the new Customize button on the top of the newly copied report.
Add & Remove Values
Expand out the Return Values section and click the blue Select Values link at the top right.
Then, click and drag any values that you want/don't want to the appropriate column and click Update at the bottom.
Change Column Name & Summary Type
Click into the value you want to modify, and then change the column label and summary type on the right-hand side.
Sum: Adds up all of the values.
Average: Returns the average of all the values.
Maximum: Finds the largest value in the entire column.
Minimum: Finds the smallest value in the entire column.
Count: Counts up the number of rows/items.
To change sorting, simply select the values using the Select Options link.
To change grouping, simply click the Update link.