VA42 user exit to update contract start and end dates at Item level

Hi  Experts,

                  I need to update contract start and end dates at Item level in 'VA42' transaction, for this i am using user-exit 'USEREXIT_FIELD_MODIFICATION' in the include 'MV45AFZZ'. i am able  to display the data into item level through my coding but the data is not getting updated into 'VEDA'

Note: No need to change contract start date and end date in header level. Only at item level i want to change dates.

Thanks,

Sateesh.