Quoted from page 94 of the DIR-615 manual:
2. What can I do if I forgot my password?
If you forgot your password, you must reset your router. Unfortunately this process will change all your settings back to the factory defaults.
To reset the router, locate the reset button (hole) on the rear panel of the unit. With the router powered on, use a paperclip to hold the button down for 10 seconds. Release the button and the router will go through its reboot process. Wait about 30 seconds to access the router. The default IP address is 192.168.0.1. When logging in, the username is admin and leave the password box empty.