This guide explains how you can convert an html confluence page to markdown
Open the sidebar, under your space you will see Space Settings
with cog icon. Go to Content Tools tab and select HTML export
radio button.
When you click export button, select custom export option and deselect all. And then in the treeview only select the required page.
For confluence server version, look for Space Tools
at the bottom of the left sidebar.
Unzip the exported space files and upload the html page to https://metamug.com/util/confluence-to-markdown/
The page may contain more html than you need. This tool automatially extracts the content in main-content section.
<div id="main-content" class="wiki-content group">
and converts it into markdown.
Copy the output and save it as an md file. Confluence allows you to import markdown file. So you can edit this markdown freely and update your confluence page later.