I do not believe that there is an Ant task for managing zip file entries in place. Even the Java API does not support incremental changes to zip files, see http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=4129445 .
However, you can simply unzip it all, rename the desired file, and then zip it back:
<mkdir dir="tmp" /> <unzip src="src.zip" dest="tmp" /> <move file="tmp/src.txt" tofile="tmp/dest.txt" /> <zip destfile="dest.zip" basedir="tmp" />