Click the small triangle on the side and select another number from the drop-down menu.

then

Then click Customize.

Then edit it yourself as required, and it's OK.

Method 2

Delete "Chapter 1", mark "Chapter 1" and select all the words to be set as the first-level title. Click the right mouse button to select paragraph settings.

At the outline level, it was selected as the first-level title.

Then delete 1. 1 and change it to "(1)", then select all the words to be set as secondary titles, right-click and set them as secondary titles as above.

or