You can credit an existing donations to a Team or switch the Team associated with a donation in the Donation settings.
- Navigate to the "Donations" tab in the main navigation menu.
- Select your Campaign.
- Find the donation you want to update and select it.
- In the donation edit page, find the Team field and select the desired Team from the dropdown.
- Click "Save"

If the donation is from a different campaign, it's a little more complicated, but it just adds one extra step:
- Go to the Donations List for the campaign in which the donation currently appears.
- Click on the donation you want to modify.
- Change the Campaign field to the new campaign.
- Click Save.
- Click on the Donation ID number at the top of the page.
- Change the Team to the one you want to attribute it to.
- Click Save.