corporate theme - changing the "Join Us Now" (/register) url

The url for the "Join Us Now" link goes to /register by default.

I need this to be a log in link rather than a register link (all my users are pre-registered).

I changed the text of the link via Appearance > Theme Options, to "Log in".

What is the best way to change the url destination. I want it to go to /wp-admin, or something similar.

I'd like to implement the change in a way that does not involve modifying the core theme files if possible.