change display mode

Hello everybody iam having problem when i change a document from display mode to change mode fields are still grey and not maintainable and i have written below code for display mode .please go through this and suggest me necessary modifications for maintaing fields in change mode.

case : sy-TCODE.

WHEN 'FPE3'.

loop at screen.

if screen-name = 'FKKOP-ZFCACHBACKREF'

OR SCREEN-NAME = 'FKKOP-ZFCACHBACKRSN'

OR SCREEN-NAME = 'FKKOP-ZFCALASTPRSTAT'.

screen-input = 0.

modify screen.

endif.

endloop.

endCASE.