Have you tried the 'download the file here' option on LW's download page?
You could try downloading the 'other' installer (click on the link below for info).
Installing using the "Other ?" installer
Or try this link (which will install 4.16.6 again).
If LimeWire fails to install
Otherwise, please go to this other forum & post your question there...the LW developers are there & since they're the ones responsible for the update warning - they should be able to fix this update problem.
LimeWire Forums