You will need to set this option within the donation form data element.
  1. Go to Fundraising > Donation Management.
  2. Find and click Edit next to the Donation Form.
  3. Click step #3 Design Donor Screens.
  4. Click Edit next to Donation Form.
  5. Select the Donation Element you want to edit and click Save Order and Edit Selected.
  6. In the Edit Donation Element box, find #3 Pre-Selected level and choose the option that should be pre-selected from the drop down menu.
  7. Click Finish to save your changes.