… instead of going through the whole document heading by heading and renumbering them all individually [rightclick on heading and track down to “continue previous numbering”].
If you need to renumber your heading manually, you did not use the built-in feature.
Automatic numbering is based on specific paragraph styles usage. If you are not familiar with styles, I recommend you read the relevant chapters in the Writer Guide.
Basically, you assign style Heading 1 to chapter titles, Heading 2 to sub-chapters, … down to level 10 (which is much more than needed in real life, but who knows?).
Chapter numbering is enabled and controlled by Tools>Chapter Numbering where you describe the properties of each level (Roman, alphabetic, numeric, …).
Caution! If you already added numbers with toolbar buttons, take care to remove this numbering before “normalizing” your outline because chapter and list numbering conflict with each other. Also, remove any manual formatting you could have applied trying to solve your problem with Ctrl+M (ideally after selecting the whole content of the heading). Note that “Restart numbering” can’t be erased with Ctrl+M; it must be done explicitly.
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