Total noob here. How do I unstack ships?

To remove all ships from a selected fleet, click the Disband button on the upper part of the fleet info panel.

To remove just some ships, click the Manage button and then use the left-pointing arrows button to remove a selected ship from the fleet. You can add ships from the same sector to a fleet this way also, whether or not they're part of another fleet.

or you can click on the x beside the ship type in the main screen after clicking on the fleet once