You can remove sites from your collections anytime. Here's how:
Go to your collection - Navigate to My Restor → My collections and click on the collection.
Find the site - In the Manage Sites section, locate the site you want to remove.
Remove it - Click the three dots (...) next to the site and select Remove from Collection.
Note: If you change a site's visibility from public to private, it will automatically be removed from all collections.