the default mapping is level 1(==) maps h2 in asciidoc https://docs.asciidoctor.org/asciidoc/latest/sections/titles-and-levels/#section-level-syntax
How to map level 1 to H1 when convert .adoc to .html?
I use an online markdown-like editor which use H1 as section title.
To achieve your goal, you would likely need to modify the converter.
Currently, the document title converts to H1. If a level 1 heading converts to H1, you'd have to figure out what the level 0 heading converts to.
You haven't stated your goal when making this change. However, if it is presentation-based, you can customize the CSS used to make an H2 look like an H1, continuing through the heading levels to making an H6 look like an H5.