The documents ID is an AUTO INCREMENT field in the database. It is automatically assigned the next available number when the document is created in the manager. In order to change the ID, you would have to manually change it in the database (using phpMyAdmin or the like), but I would strongly advise against that as it will interfere with document retrieval and even worse, it could potentially affect your ability to create new documents.
To get a document’s ID in the content or in a chunk, you can use [*id*] for the current page. If you want to make a link the current page, like to an anchor in the page, you could use
<a href="[~[*id*]~]#anchorName">Go to the anchor</a>
As Zi said, you can make a link out of any document if you know using it’s ID. To link to the default site start you could use [~1~] as it’s document id in the default content is 1.
Perhaps you could elaborate on why you want to change the ID number for a document. An explanation would help us understand your needs and then be able to provide you with a more specific answer.
-sD-
Dr. Scotty Delicious, Scientis.