First, figure out which project you need to change.
SELECT id, title FROM projects;
Next you need to know the id of the user or group you want to assign the Project to.
SELECT id, name FROM groups;
SELECT id, login FROM users;
Then, update the projects table accordingly, replace GROUP_ID/USER_ID and PROJECT_ID:
UPDATE projects SET owner_id = GROUP_ID, owner_type = "Group" WHRE id = PROJECT_ID;
UPDATE projects SET owner_id = USER_ID, owner_type = "User" WHERE id = PROJECT_ID;