Change Column Name in a report
There are a couple of ways to do this - but the easy no-coding way is to use the "Word Replacements" function.  Go to Administer menu > Customize Data and Screens > Word Replacements and put in the phrase you want to replace, along with what to replace it with.  Don't forget to check "Enabled", and next time you reload that report page, you should see your change.

For the official documentation, check here: http://gitbook.civicrm.org/initial-set-up/customizing-the-user-interface.html