You have to go to where your old ship is stored
Ok, and what if i forgot where i left it? :S
Can i find it on the Galaxy map somewhere?..
Yes a ship symbol shows over the system.
If you don't want to leave your current ship way back there you can buy a Sidewinder for 37K ishand use that to run back and grab your other ship.
The galaxy map will have a little red ship icon over the system that has your extra ships. If you're having trouble reading that go to any shipyard and go to the far right panel and it will list stored ship locations. Once you get into the system, the system map will show a blue ship icon over the station that stores your ship