First, you need to make that field editable. While building the field catalog for this field, set the EDIT parameter to 'X'.
Next, you capture the changes made to this field in the ALV display when user clicks on, say, the SAVE button. To do that, use the i_callback_user_command parameter of the REUSE_ALV_GRID_DISPLAY FM (assuming you are using GRID ALV).
Give the name of the form where you will handle the user action (in this case SAVE).
i_callback_user_command = 'F_USERCOMMAND ' .
Next, write the logic in the F_USERCOMMAND form.
Sample code:
FORM f_usercommand USING r_ucomm LIKE sy-ucomm
rs_selfield TYPE slis_selfield.
CASE r_ucomm.
WHEN '&DATA_SAVE'.
*logic to save the changed data in the form.*
Here, call the internal table which you have used in the T_OUTTAB parameter of the
FM REUSE_ALV_GRID_DISPLAY. It will contain the updated data from the ALV.
ENDCASE.
ENDFORM.
Check below links:
**************** - ALV with EDIT and SAVE functionality
ALV with edit and save functionality - Code Gallery - SCN Wiki