Google-docs – How to modify paragraph styles in Google Docs

This will only work with Headings:

• Locate an instance of a heading you want to modify
• Modify it in-place (e.g. select and increase the font)
• Then, while leaving text cursor inside the heading, open Styles menu in toolbar.
• Find the heading you want to modify in list, click on the arrow submenu.
• Select entry "Update Heading X to match".

This will update heading style for this document.

You can then go down the Styles menu, open Options submenu, and choose "Save as my default styles". Now, every new document you create will use this style you have just updated.

You can then go to existing document, and in the same submenu choose an option "Apply my default styles" -- and then your modified styles will apply to this existing document.