It is probably dead easy if you have the vaguest idea of how to solder. I would use a bit of heatshrink wrap too, that can make the bodgiest of soldering jobs strong enough to stay together. If you're joining up cables, make sure you get the wires connected to the right one in the other cable, or else your battery / light might blow.
If it's actually inside the light rather than just bodging something in the cable, that might be a little harder.
If you don't already have a soldering iron, personally I'd recommend just finding a mate who solders and giving it to them. It'll take you an hour of faffing around, not to mention buying a soldering iron, whereas someone experienced can probably do it in a couple of minutes (or at least that's my experience of being a not very good solderer).
Joe