How can I edit site pages URL (example.com/page1.html)

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2

if you have access to your server (not using bss server) then you can change with htaccess

RewriteEngine on
RewriteCond %{THE_REQUEST} /([^.]+)\.html [NC]
RewriteRule ^ /%1 [NC,L,R]
RewriteCond %{REQUEST_FILENAME}.html -f
RewriteRule ^ %{REQUEST_URI}.html [NC,L]

Or you can change the file structure as outlined here: