I assume the fitting you want to modify isn't already adjustable (eg two parts that slide one within the other)
If you tackle the top of the tube, apart from the business of cutting it, the tools needed will surely depend on how the fitting is constructed, and also how you propose to reassemble it.
If the tube is currently threaded at the top, and held up with a nut, after you've cut it to the new length, you might decide not to thread it, but to use another method.
You might find or make a collar with retaining screws to fix on the end of the tube, or drill a hole through one side of the tube and out the other, and hold it up with a split pin, or bolt, or a stud through the hole, or even slice the tube vertically in pairs of cuts close together and bend tabs outwards like 'wings'.
On the other hand if you have a set of dies that include the thread of the original fixing, you can put a new thread on the end!
If it's chrome plated you might have to remove at least some of the plating before trying to cut the thread.
It would be good to remove the flex within the tube first, which presumably means disassembling the bottom part of the fitting first. So I wouldn't say it was 'easy', but if you plan what to do first, it should be 'straightforward'
Suggest you need at least a hacksaw and a file, and if you don't already have suitable dies, I would abandon thread cutting and use a different method.
n this photo.
Cheers........
