The easiest way would be to connect you Pi to a monitor and a keyboard.
Turn it on and configure it directly from the console.
Should you not have a keyboard or a mouse around (e.g. you only have a laptop), you could change your computer settings so that it will see the Pi.
For example if your Pi is on the 192.168.1.x network instead of 192.168.0.x, just change your laptop network settings to see the 192.168.1.x and give it an address that is different from the one you gave to the Pi, and enter via ssh. This requires cooperation from the router though.