Your Pro payment entitles you to free updates for up to 6 months. So, if you bought Pro less than 6 months ago from the official site www.limewire.com then go here & enter the email address that you used to buy Pro.
http://sales.limewire.com/support/?

If you bought Pro more than 6 months ago, or from a scam site, you won't be able to update & you'll have to buy again if you want Pro

It's a good idea to always save & back up your Pro installers.