If you've asked someone who isn't affiliated to a registered school/college to write the reference and it hasn't been completed, you would first need to click on the Reference section and then the option to 'Ask a different person to write reference.'
If you've asked a registered school/college to write the reference and it hasn't been completed, you would first need to ask the school/college to decline your request to be linked to their institution. Then you can proceed with choosing another referee. If you want another registered school/college to write the reference, you can click on Options and then 'Ask a registered school, college or organisation to write your reference only'.
If you want someone to write the reference who isn't part of a registered school/college, you should click on the Reference section and complete their details there. Then you should click on 'Ask referee to complete reference.'
If the reference has been completed either by a registered school/college or by a referee who isn't part of one, then you wouldn't be able to change the referee details. In this case, you would have to register a new application and complete that one instead.