Change your password
Change it under Settings, Security. Your other devices are signed out; the one you use stays.
Under Settings, Security, enter your current password and the new one twice, and tap Update password.
Your other phones and computers are signed out; the device you did this on stays signed in. A new email address under Settings, Account asks for your password as well.
An admin can set the password of someone else under Users. That signs out all of that person's devices in the same way, which is the thing to do when a phone is lost.