Do the following;
Talk to Quinn in office (you are told to do so)
Go to the doors above shipyard. Talk to the woman behind the desk and you can select any one class of ship). If you want ot puirchase the other 2 talk to her again and tey will cost ~20k energy credits.
Go down to shipyard and talk to the guy on the right as you enter 'select ship'.
Choose your new ship as current.
Then talk to anyone about customising it and there you go.