I have a word document that looks like this. When creating the multilevel list, I forgot to include the Performance, Ecosystem, Popularity, Summary chapters to the list. How do I append them now so that Performance would be 3.1.6, Ecosystem 3.1.7 and so on..? (React, Angular, Vue, Benchmarks would be subheadings for Performance)
Assuming that you have set up the Multi-level list with Level 1 linked to the Heading 1 style, Level 2 linked to the Heading 2 style, Level 3 linked to the Heading 3 style and level 4 linked to the Heading 4 style, you would apply the Heading 3 style to the headings of the Performance, Ecosystem, Popularity, and Summary chapters, and the Heading 4 style to the React, Angular, Vue, and Benchmarks.
If that is not the way that you set up the Multi-Level list, and you need to know how to do that, see the article "How to create numbered headings or outline numbering in your Microsoft Word document" on the following page of the Late Shauna Kelly's website: