Фиг ее переназначишь.

Открываем документацию, глава "Your Nokia E51", подраздел "One-touch keys" (страница 26 в английском мануале):

One-touch keys

With the One-touch keys you can access applications and
tasks quickly. Each key has been assigned an application
and a task. To change these, select Menu > Tools >
Settings > General > Personalisation > One-touch
keys. Your network operator may have assigned
applications to the keys, in which case you cannot change
them.

1 — Home key 2 — Contacts key 3 — E-mail key 4 — Calendar key