As long as the multi-level heading style is defined, when you set the heading of 1.3, 1 will automatically come out.

1. Define multi-level titles according to preferences. If there are four levels of titles, you can set them in a similar way:

2. Select the title1.2.1~1.2.3, and then select "Title 3" in the style bar to set the title style.

Now I want to change 1.2.3 to 1.3. 1:

3. At this time, 1.2.3 must be preceded by the title of 1.3 to be logical, because 1.3. 1 must be the first section below 1.3.

4. Select the title 1.3, and select "Title 2" in the style bar.

5. So 1.3 came out, and the corresponding 1.2.3 became 1.3. 1.