“How to change the user's password.”
Every user has its own file in your users folder. The filename is an sha512 hash of the lowercase version of whatever the user’s name is. Inside the file you will find an sha512 hash of the user’s password (salted with the user’s name hash).
As the forum admin you have access to the files on the server and can just swap out the hash with a new one.
As a hint, there are plenty of websites that can help you hash strings. DuckDuckGo even has it built-in: https://duckduckgo.com/?q=sha512+Test