How to update infotype screen field value

Hi i am working on enhancing infotype 0185 and i am supposed to read the value of  screen field 'date of issue' that the user enters and accordingly calculate the 'valid until' screen field value. So far i was able to retrieve the screen field value using CL_HR_PNNNN_TYPE_CAST=>PRELP_TO_PNNNN

and calculate the 'valid until' value what i am unable to do is update the value of the screen field 'valid until' with the date i calculated.

i tried using the following code:

CALL METHOD CL_HR_PNNNN_TYPE_CAST=>PNNNN_TO_PRELP

     EXPORTING

       PNNNN = WA_USER

     IMPORTING

       PRELP = INNNN.

* WA_USER is the workarea i used to retrieve the screen field values using PRELP_TO_PNNNN, i also added the newly calculated date into it.

Any help would be greatly appreciated