Hi
Thanks for your question.
When the noun "content" is used to refer to an amount of something within something else as is the context of your example sentence, it is always uncountable. So you should say "New content will be updated regularly." Here's another sentence "New content is regularly added to Workplace English Training E-Platform."
The plural noun "contents" has a different meaning altogether. It is used to refer to something within a container, such as items in a bag, desk, house, etc. For example: "The contents of his bag were scattered across the floor."
Regards
Dr English