To edit on the report:
  1. Right-click the formula field on the report
  2. Select Edit Formula
  3. Make the desired changes to the formula
  4. Click Save & Close
To edit using Field Explorer:
  1. Open Field Explorer (click on icon with dashes in the upper toolbar)
  2. Click the + sign next to Formulas
  3. Select the desired formula to edit
  4. Right click and select Edit
  5. Make the desired changes to the formula
  6. Click Save & Close