A community can be set up in sevaral ways but in the default setup any colleague can edit a published Knowledge article. All colleagues have author permissions for the knowledge pages in the tenant.
This is how it's done:
- Open the knowledge article.
- Open the editor and select "Write".
- Do the changes and save.
When editing you can do all that is available when creating a knowledge article, see: Create knowledge article </using-communities/knowledge-articles/create-article/index>