Today we're release ezQuake 1.9.1 stable. This release fixes three issues from version 1.9, besides these, it's completely similar to 1.9.
Read the changelog for further info.
To upgrade from 1.8 to 1.9.1 you need to install whole package.
To upgrade from 1.9 to 1.9.1, just copying over the new ezquake*.exe file is enough.
[ http://ezquake.sf.net/docs/?changelog ]