Ask user to use Forgot Password link located under login form. If that is not possible and you have to reset password follow this process:
The only way to reset user's account password is to remove account and recreate it in the system.
- Use filters to find user's account
- Click on Delete link next to user
- Click on Create new button to create new account for user.
- User will receive email about acount created.
- Click on Send Activation link to the user.