I want to replay the Dragonborn DLC.I've already finished it.I don't want to make another character.How do I do this?

Load an older save.

If you don't have an older save, then you can still visit the island. But if you want to do the Main Quest again, and don't have an older save, then you are going to have to start a new character.

Load an older save, or start a new character.

Remove Dragonborn from a save and you WILL utterly destroy that character.

What would happen?

You cant cleanly uninstall a mod, and reinstalling that same mod will not make it any better. Im not sure of what it does exactly, but there will always be parts of that mod left over, especially with large mods with scripts, and eventually it will destroy the game for that character. DLCs arnt safe either, as theyre just an official mod. Im not sure if its true for consoles, but i wouldnt risk it. As a good rule of thumb, never uninstall a mod and continue playing on that same character. If you remove a mod, youll need to load a save from before you had that mod, or make a new character.