There is no simple function built-in to do that at this time, however there are 2 ways the admin user could do what you need:
1. Admin can clear the sign-up, and then use the live search feature set to search the WP Users table, and sign-up the same person again (clear from the backend, then signup from the front end, using live search and selecting the user).
2. Can directly edit the sign-ups table, find the entry for that sign-up, and edit the user_id field, entering the user id (number) of the user’s account. Super easy to do with PHPMyAdmin, or similar server control panel database interface.