How can I refund money to client?

Here's Guru's explanation for issuing refunds for invoices already paid and funds released to you:

http://www.guru.com/help/freelancer-help-center/get-paid/guru-invoicing/#how-do-i-issue-a-refund-to-an-employer

All you have to do is go to "Manage Safepay Requests" under "Payments".

Locate the client's payment and on the right side you will see a link "issue refund". Enter in the amount you're refunding (full or partial) and it will go back to the employer once you submit it.