Selection Screen

Loop at screen
modify screen.
endloop.

To modify screen outlook.

http://help.sap.com/saphelp_nw04/helpdata/EN/9f/dbab6f35c111d1829f0000e829fbfe/content.htm

eg:

REPORT demo_dynpro_modify_simple.

DATA: ok_code TYPE sy-ucomm,
save_ok TYPE sy-ucomm.

DATA flag(1) TYPE c.

CALL SCREEN 100.

MODULE status_0100 OUTPUT.
SET PF-STATUS 'SCREEN_100'.
LOOP AT SCREEN.
IF screen-group1 = 'MOD'.
IF flag = ' '.
screen-input = '0'.
ELSEIF flag = 'X'.
screen-input = '1'.
ENDIF.
MODIFY SCREEN.
ENDIF.
ENDLOOP.
ENDMODULE.

MODULE cancel.
LEAVE PROGRAM.
ENDMODULE.

MODULE user_command_0100 INPUT.
save_ok = ok_code.
CLEAR ok_code.
CASE save_ok.
WHEN 'TOGGLE'.
IF flag = ' '.
flag = 'X'.
ELSEIF flag = 'X'.
flag = ' '.
ENDIF.
ENDCASE.
ENDMODULE.