Answer by HeyJude for Is it possible to mark all articles in a group as "unread" in Feedly?

Scroll down to load all articles inside the feed you want to mark as unread, press F12 to open the Developer Tools Console, paste the following code in the console, and hit enter (source):

document 
 .querySelectorAll('.mark-as-read') 
 .forEach(markAsReadButton => { 
   if (markAsReadButton.title === 'Mark as unread') { 
     markAsReadButton.click() 
   } 
 }) 

It works, but you have to remember that Feedly, at least on the free program, only saves articles 30 days old, so only these will be saved as unread.