From the link I provided:
Having purchased a pre-built ship you can deploy it from any Shipyard.
For this go to the "Local Services" at a port and select the "Shipyard" button:
All purchased pre-built ships will be displayed on the right hand side for quick access. Vessels ready to be deployed will have the "Ready to Deploy" text towards the bottom of the image and a yellow icon in the top right:
Select the pre-built ship to deploy, in this example we will use the Type-6. This will display the ship preview for this vessel and a central "Deploy Pre-Built Ship" button. Selecting this button will then deploy the vessel to your current location:
Following deployment you will be presented with a notification to inform you the vessel is now ready for use in the bottom right:
Simply select the now deployed pre-built ship to be shown the ship management options:
Select "Use This Ship" and your current vessel will be stored and you will be transferred into your pre-built ship! Congratulations you are now ready to experience your pre-built ship!