Or you can opt for the more efficient solution and create and apply a numbered style.Unfortunately, that can get you into even more trouble, if you don't know what you're doing.Most everything you need is built in and can be easily modified.
All you have to do is add a numbering scheme to the top heading style and watch the settings trickle down. This is where you'll define the numbering scheme for Heading 1 as follows: click OK — you still have to define the numbering style for the sublevels.
Working with an existing document and all its in-use styles can be troublesome.
It can be done, but changing styles that are already in use almost always has far-reaching repercussions.
If you want to update these items, the shared functionality serves as a convenience.
No need to update cross-references, tables of contents and separately; just select the whole document and click Update Field.