A secured document is automatically deleted after its valid period has elapsed. How to change the valid period is described here.
Select <Menu> in the Home screen.
Select <Function Settings>

<Secure Print>.
Select <Secure Print Deletion Time>.
Change the period of time until which a secured document is preserved in the memory, and then press <Apply> (

).
When using a touch panel model, select <Apply>

<OK>, turn OFF the machine, and then turn ON the machine again after 10 seconds.