put https://{subdomain}.pinpointhq.com/api/v1/custom_attributes/
You can update the value of a Custom Attribute.
To do that fill in the _value field corresponding to the field_type of the associated Custom Field (more on this in the Create Custom Attribute docs). Leave the other _value attributes empty.
Limitations
You can't update the custom_field nor custom_attributable. In case you want to reassociate the Custom Attribute to a different Custom Field or resource please delete it and create new one.