Update product inventory level

[Lubard]

Hi,

This may be a dumb question, I want to update product inventory level using incremental value instead of overwrite it.
I want to do this in one connection.
I tried several ways of Bigcommerce::Product.update(xxx, inventory_level: xxx)
They all overwrote the inventory level, or I have to make another request to get the current value and overwrite it?

Another one,
I keep receiving

Error Faraday::ConnectionFailed Happened
Hostname not known

Even though I updated the gem to its latest version.

Any ideas?

Thanks in advance.