You can modify your longship at the shipyard in your settlement.
The shipyard can be built once your settlement reaches level 2 through UPGRADES and becomes a colony.
How to Modify your Ship
To modify your ship:
• Locate the shipyard in your settlement.
• Talk to Gudrun.
• Choose See Shop.
The shipyard menu will open.
• Select the Hull, Sail, Shield, Figurehead or Tailpiece slots.
• Pick an available option to change the look of that part of the ship.