Admin login access

How can I find out about and change the the main admin login information IF I don’t know it? I currently have limited access to my WP CMS, logining in thru a different login than the main admin user??

The reason I ask is because we can not get hold of the person who set up our WP account and is the only one who know the main admin login information.

Please help so we can continue to manage our website.

thank you