[Resolved] Slug nb to no

Background of the issue:
I need to change the language code for the Norwegian pages on my site hidden link from /nb to /no. I have never heard of anyone using /nb/. I am following this documentation: https://wpml.org/faq/how-can-i-assign-or-change-language-code-to-existing-content/

also: hidden link

Symptoms:
The language code for the Norwegian pages is wrong. WPML says that: The Language name already exists.

Questions:
How can I change the language code from /nb to /no for the Norwegian pages?