You can definitely edit the unsubscribe link.  An unsubscribe is necessary, so if you do not include your own on your message, the Email System system will automatically insert one at the bottom of your email.

To add your own link, just apply the text (e.g. Click here to unsubscribe) or unsubscribe graphic and click the Insert/Edit Link icon from the Visual Editor tool bar (it's the 10th one from the right in the middle row, and resembles pieces of chain linked together).  A window will pop up asking you to enter the link, but you will also see a drop-down menu labeled Special URL.  Just select Unsubscribe URL from this menu and click the Insert button.  The link will now be in place, and the default message will not appear at the bottom.

If you are manually applying the link in HTML, you can use the following value:

<a href="{{{unsubscribe}}}">Place your unsubscribe link text here</a>