Click under "My loans" on your active loan: 

Select "Manage loan":

Look for "Change payment day" and from the drop down list select the payment day you would like to have:

For example 15th and "Confirm change":

NB! There has to be done at least one payment in a month. If you would like to change your payment day from the 1st to 15th on the 8th day of the month, it would be fine, but if you would like to change the payment day to 15th on the 28th day, the new payment schedule would not start from the next month, but from the month after it. It is possible to choose dates between 1-28 as all months in a year have at least 28 days. If your loan has debts, it is not possible to change a payment day.