you need some torx bits to undo the selector mechanism bolts, I forget which size though. It's just the bolts on the outside of the box.
You'll need a gear puller too if you intend on taking the shafts out, fifth gear has to come off because it's on the outside of the casing under the black cover.
I'd be tempted to just take it to a specialist. Or at least get hold of a cheap/free knackered 5speed polo box to practise on first (that's what I did), taking the innards of the box apart isn't for the faint hearted! I've got my old ATV sat in a box too, it wont go into 1st gear. But I'm not doing anything with it, it's waiting to go to a proper gearbox rebuilder.