MyBB Community Forums

I have an old forum (1.6) that I want to upgrade to 1.8 due to security and new features.  I moved everything to a new domain and host (1.6), installed the database so it's close to being a "clone" site.  I downloaded 1.8, replaced all the files through FTP on the server (based on what the mybb upgrade instructions say to do).  

The issue seems to be when I go to install/upgrade.php, it says the page isn't working. If I go to /install or the main website directory, it treats it as a brand new installation that will overwrite the old (1.6) database. What step am I missing?  I just want the users, threads, and posts to be showing then I'll deal with plugins later.  I just can't get to the upgrade for some reason.  What am I msising?

Can you screenshot the error you see? Sounds like a browser/server error more than a handled MyBB error.

Okay, I was able to get to the "upgrade process" screen where it asks me to login; however, when I put in the username and password for the admin, it just "loads" without going anywhere. I put in a wrong username and password, to which it informs me it is incorrect. When I put the correct one, it just refreshes with empty boxes at the username and password and goes nowhere.

I know it's the correct username and password, as I logged in on the previous version of the site on another domain and it worked fine. BUT if there is something wrong, how would I find out the username and password for the backend? (Which file can I look at to see it?)

You should upgrade in your old host (domain) and then migrate, or migrate your old site and then upgrade.

Your issue sounds like a cookie issue.
https://docs.mybb.com/1.8/faq/login-problems/