To update Gnutella (as well as everything else that needs updating), copy/paste this into a terminal:
Code:
sudo aptitude update && sudo aptitude safe-upgrade
Generally, you'll want to answer yes to what it asks you.
What's a terminal and where do you get it you ask? Simple. A terminal, aka a console, let's you enter text commands that work the same (or better) than commands issued in a GUI. You can get a terminal by going to Applications >> Accessories >> Terminal.
If you're more comfortable in a GUI setting, you can open up Synaptic and search for gnutella, (synaptic has it as gtk-gnutella) right click and select "Mark for Upgrade". If Synaptic doesn't show gnutella as installed, simply click on the box, then click apply.