You can download the latest firmware version for your DIR-615 Revision C by clicking the link below:
http://www.dlink.com/us/en/support/product/dir-615-wireless-n-300-router?revision=us_revcNOTE: If your router is working with out any issues, it's recommended to keep the current version of FW that is loaded.IF IT WORKS, DON'T FIX IT!!! 
Do
NOT upgrade firmware on any D-Link product over a wireless connection.
Failure of the device may result. Use
only hard-wired LAN network connections.
Please follow the (link)>
FW Update Process for upgrading.