Click on the ship you do not want in your fleet and click eject in the fleet's window at the bottom left, when you've selected the fleet of course. This will move it out of the fleet in the same hex. To decommission a ship there is also a button in the same window if you have a ship selected. Click manage then decommission. You should be able to select the ship in the fleet and decommission it while its individually selected. To eject multiple ships hold ctrl and click each one. Then eject. To decommission an entire fleet double click the fleet. In the display that appears there should be a decommission button in the bottom left.

Click on the fleet in question.  After selecting it, in the lower left corner you will see the ship icons of all ships in the fleet.  When you select one or more of these icons, you will see a list of options just to the right of the icons.  One of those options will be 'eject.'

Good luck with your gaming.

Thank you!  My ship icons were too small for me to see, so I didn't notice them in the right-hand panel at the bottom left.

The screen, yes.  The sub-panel that shows individual ships in a fleet, no.