At first, select some content in OneNote UWP which you want to save as HTML file.
You can use keyboard shortcut Ctrl+A to select content.
Press Ctrl+A, OneNote UWP will select a paragraph.
Press Ctrl+A again, OneNote UWP will select all content in a container.
Then, click "Gem Menu" -> "Gem" menu -> "Save as" menu item.
It will pop up a "Save as" window to choose where to save the .html file.
Input the file name to save the HTML file.