Domain user password

So I am helping a gym I work at with some basic computer things like troubleshooting and what not. We all get a username and password for the domain so we can use any computer in the building. A girls account wont let her in and her password she swears on isn't working. I have access to her local computers admin as well as the admin for the building. He gave me his user name and password when he quit and its still active. So I have access to everything just need to know how to change a domain password or just create a new user account on the domain its self. Any help would be awesome. thanks!