Can we dismantle fleet ship and can we upgrade ship (especially purple) to Legendary; If can, how to..

You can dismantle some fleet ships. The ones from each season that you can build with parts. You do this in the Ships menu. If a ship is able to be dismantles you will see a Trash icon by the name.

You cannot upgrade them to a higher class but you can keep trying to build new models of these particular ships to tr to get a better version.

Other fleet ships are only available in 1 set class.