locked out of my sites

I wanted to get in and update to a newer version of WordPress. I attempted to use the login and password I remembered a few times but it said it was wrong. I have 2 sites with the same basic domain name. I attempted to log into the other site (different username and password) but as soon as I put wp-admin in the URL it told me immediately that I had entered wrong info and wouldn’t even let me attempt to get in.

I have 2 problems here:

1. How can I find a way in to the site whose login information I know for sure?

2. How can I change the login information of the other site? Here there is a secondary problem: when the site was set up, a dummy email address was used because at the time, we were just playing around with it. So there is no legitimate email address attached to that site. Help please!!!

Thanks!

Jane