How Update Customer Account Activity Credit Memo Application Flexfield using API Rest or SOAP

Summary:

Content (please ensure you mask any confidential information):

When we have a credit memo created using the commands Action -> Credit Transactions, the Credit Memo becomes a Customer Account activity and can't be update via Api Rest or Soap.

We want to know if we have another option to update a flexfield field, preferably using API Rest, but if is not possible, using SOAP.

Version (include the version you are using, if applicable):

23D