While Mark’s answer is correct for Profiles, it seems like Stevie is talking about Consolidation. Unfortunately, there is no direct API (or otherwise) to update Master Attribute Data on an Identity in Consolidation.
I wrote a post here on the available operations with the Consolidation API: NERM Consolidation API Usage - Identity Security Cloud (ISC) / ISC Community Knowledge Base - SailPoint Developer Community
The Master Attributes are fed by the values the Master Identity’s matched Data Records. So, you should be able to update the Data Records to have a new value to feed into the Master Attributes. That would be done by re-importing the record with the changes you need.
One caveat to this is that you can not send in a value as “blank” - as the system will ignore blank values during the matching operation. So, you would need to send in some kind of value to wipe it out. That could be a string like “null” or a hyphen ( “-” )