To edit the label for the donation level that appears on the donation form you will need to edit the donation form itself:
  1. Select Fundraising > Donation Management
  2. Select All Donation Forms
  3. Locate the correct form in the list and click Edit
  4. Click Design Donor Screens
  5. Click Customize next to Donation Form
  6. Select the donation level element from the active list (right side list)
  7. Click Save Order and Edit Selected
  8. Edit the label next to the donation level on this screen
  9. Click Save

After saving your changes, you should be to view these changes on the front end of your site. You may also need to clear your browser cache or open an incognito window to view these changes.