Can't Login using wp-admin

I set up a website using WordPress for my son 2 or 3 years ago. I remember something about wp-admin being a security issue so I think I somehow changed the admin login to something else and now we can’t get in. Help!

Is there some PHP file we can edit to get this back to just wp-admin?

I don’t know if I did this manually or used a plug-in but I don’t see a plugins directory. I’m a bit confused, I’m one of these people who is great at figuring things out when I have the time, but then after a couple years I have no idea what I did. Should have kept it simple!

Thanks.