1. Click Reports in the menu
2. Click Manage Reports
3. Click on the category you stored your report under (ex: System)
4. Click on the name of your report to open it
5. In the Reports column section at the bottom of the page, highlight the name of the field you wish to rename:
 
Highlight the Field
6. Type in the new label you'd like to assign:
 
Assign a New Name
 
7. Click Save And Run on the left, and then proceed to rerun your report.

Important Note: When you rename a report column it can be easy to forget later what the name of the original field actually was. If the name on a report field has been changed and you can't remember what the actual field is, you can click on the expand icon next to the field on the Edit Report page, and then look at what is listed for the Details. Using the example above, if we click on the expand icon next to Donor, we can see in Details that the original field was the Account Name field:
 
Field Details