You can't, but you can create a new account and import your exported bookmarks from your old account.
From the Delicious FAQ:
How do I change my username?
There's no easy way to do this right now, but
you can use the export and import
features in your settings to change to
a new account while preserving your
current bookmarks. First, visit your
Settings (accessible from the top
right of every page on Delicious) and
select export/backup from that page.
Follow the directions to export your
bookmarks. Then, register or log into
your new account, go to settings, and
select import/upload. After importing
your bookmarks into your new account,
you can log out of it and log back
into your old account to delete that
one. To do this, go to Settings and
choose delete account.
