The best option would be to configure your web server (IIS, Apache, etc.) so that a request for the old page sends a 301 Redirect to the new page. This will preserve your search engine rankings. A 301 Redirect means the page has moved permanently. For example:
www.example.com/infopage.html renamed to: www.example.com/infopage.asp
You would no longer have any page available at infopage.html. But you would configure your server so that a request for that page would send a 301 Redirect to the new page.
If your site is hosted and you don't have control over the server, you may need to work with your web host to get this configured.