How to login to pagespeed admin or change login password

I’m having a caching issue where I am unable to refresh the style sheet on my WordPress site, I’m assuming for now it’s to do with Pagespeed.

The problem is that having found out how to clear the cache (e.g. navigate to I am prompted for a password but there is no clue anywhere as to what login details are required (WordPress login or server login details don’t work).

Where do I find this info or is there any way I can set up a new login for Pagespeed?