Laybuy does not allow payments to change to fortnightly installments. However, you can pay your individual installments off in advance at no additional cost.
You will need to log into your account here: https://www.laybuy.com/consumer#/ Once this is open, you will see all upcoming and overdue payments that you currently have with Laybuy. All you need to do now is select the order you would like to pay more of.