The current password change/reset in Flarum is currently a single feature re-used for both password change and password forgotten, as it reduces the amount of code needed. So unfortunately Flarum doesn't have any way to change password without emails natively.
It shouldn't be too complicated to implement this as an extension. Admins already have the ability to change anyone's password, it would just have to be the same code with a check for actor==user and optionally check for a password to be entered. The logic for current password confirmation could be copied from email change.