ClamAV and F-prot can be updated from within Parted Magic.
Read the help text (Press F1 and F2 when you are at the first screen of UBCD).
Avira antivirus can't be updated on the fly for the moment, because Avira doesn't support it currently. Maybe it will support it in the future.
Post a request at: http://forum.avira.com/wbb/index.php?page=Board&boardID=210
But don't expect that it will be added soon, it was already requested some months ago, but it isn't added yet, just like the other feature requests.
Download Ultimate Boot CD v5.0
Use Parted Magic
for handling all partitioning task: http://partedmagic.com/