As the Admin, you can quickly reassign donations.
Go to the Donations tab.
Scroll over the donor name.
Click on the pencil icon and click "Reassign Donation".
Then select the participant name from the drop down menu.
If you don't see the participant name, then you might need to create a pledge page for the participant. You would add them on the Groups & Participants tab first, then you'll be able to reassign.
Note: Splitting donations is not possible. We can refund the donation so that the donor can go back in to redo the donation as desired. Or you can reassign the donation to one of the participants.