If you bought Pro more than 6 months ago, or from any site other than Lime Wire » Download LimeWire then you can't. LW Pro costs $18.88 & entitles you to free updated & email support for up to 6 months.

If you did buy Pro from the official site, less than 6 months ago, go here & enter the email that you used to buy Pro.
http://sales.limewire.com/support/pro_lookup.php

Or follow the link to your Pro personal download page (that you would've recieved when you bought Pro)