This document provides an example on how to clear the "Date Of Birth "field value by OData API.
For example, User contains below information:
You may use upsert operation and set "dateOfBirth" field value to null, as below:
Please also refer to the OData API reference guide for SAP Successfactors HCM suite:
Date Of Birth; dateOfBirth; OData API; Date time; User , KBA , LOD-SF-INT-ODATA , OData API Framework , How To