Customers can reset their own password on the storefront by following these instructions or you can help them as an admin.
Navigate to customer that you want to change their password for under People > People and click Edit.
Click on General Info on the lefthand side.
Scroll down and you'll see the option to set a new password.
You can also send a user a reset password email or obtain a reset password link for the user and provide that to the user directly.