Updating master product Spree Rails api

To update a product using Spree API we have to make a request like:

PUT /api/products/a-product?product[name]=Headphones

This works for me the product is updated, but when I pull the product through API, the name for product is updated but not that for the master.

Then I tried something like this:

PUT /api/products/ruby-on-rails-tote/variants/2?variant[sku]=12345

thinking master might be a variant itself, but this does not update the master product. Am I doing something wrong here? What's the correct way to update the master of a product in a Spree store through API calls?