Order edits allow runners to make changes to their own registration, please see here: https://help.ultrasignup.com/guides/event-setup/enable-distance-changes/
If you do not want to enable order edits, you can change a runner’s distance manually.
To open your participant record to edit it, simply click on the edit link next to the participant in your list.

You can also search the participant in the search box. Clicking on their name will open their record.

Scroll down the participant record to the dropdown box labeled “event”:

Simply choose a different distance from the dropdown and click the green “update” button.
Don’t forget to refresh your participant list once you have completed this step. Please see the instructions here: https://help.ultrasignup.com/guides/registrations/refresh-your-participant-list/