update not allowed for field 'Party ID(PartyNumber) in D365 AX

Hello guys,

I have imported 1000 customers in D365 and its looking fine. In these customers cash discount field value is blank.

Now need to update all customers with Cash discount field , so when trying to import with cash discount field system is generating below error.

Results. update not allowed for field 'Party ID(PartyNumber)'
Results. Validations failed

Please suggest how to update fields.

Thanks.