Change from address of user registration email
There is one e-mail address that is used as From-address for all mails sent automatically by the system.  By default, this is set to the same as the site administrator email.  To change it, do the following:


Navigate to Administration » Configuration » System » Site Information.
Locate the field "E-mail address". Change it to whatever you want it to be.
Press Save configuration.


Note that will also be used as the From-adress for password resets and other generated mail.

If you require fine-tuning beyond this (to distinguish between different types of auto-generated email messages), you need to write a custom module. In that case, you may use hook_mail_alter to alter the From:-address, based upon some criteria (e.g. the subject field.

For avoidance of misunderstanding: The three steps listed above will not change the site administrator email id.  To inspect and change the site administrator email id, click on the  "My account" link while logged in as site admin, and then press the Edit tab to see it. The  site administrator email id is by default the same email id as the From-address used for email sent by the site itself - but they do not need to be the same.