As stated in the HT Reference Manual, you'll need a 16mm Deep Socket wrench to remove the tires. (Alternatively, I am told that a six-sided 5/8" sparkplug socket works as well, but I've not tested that option.)
Make sure it is "deep socket", meaning "tall". Also make sure it is a 6-sized opening, as there are other sizes. To see what is "six-sided" mouse over the photo to the left.
To
remove and replace the handlebar, I've chosen to use a T-Handle hex wrench,
rather than the "standard" L-shaped hex wrench style that came
with the Segway. I feel they are more substantial, and don't get lost
so easily. At sears.com, you can choose the Craftsman 7-piece hex key
set, Sears item #00946298000, which is also referred to as Craftsman Model
#46298. The price is $19.99. The photo to the right is the Craftsman set.
You'll end up using the 3.0mm hex for the battery, and the 4.0mm hex for
the handlebar. The item depicted above, that I purchased from Home Depot,
is the Husky set, available in the tool corral. It's a $19.97 set also,
and comes as a part of their SKU# 637634122411.
With a deep-socket wrench, 3/8" drive, you'll have the clearance you need to get the nut off.
From the side, you see here that you have the clearance:
If you use a short-socket wrench, here's what the problem is:
And no clearance to properly move the wrench:
The one way to rectify this problem is with an extention:
To properly tighten your tires, you'll need a torque wrench.
There are several types. The one I've chosen, below, is a drive-style,
meaining it is a combination torque wrench AND a ratchet (seen in earlier
steps). 
Here is a detail of the wrench handle. According to the
manual, you'll need to set the torque to 50 N-m (which stands for Newton
meters, a torque measurement). By dialing the handle up to 44.1, and rotating
the handle to 6.1, you get 50.2 N-m, the setting you need. Tighten until
the ratchet head begins to click, indicating you are at the proper amount
of torque on the nut.
Next,
re-seat the tire on the platform, and begin to tighten the wheel nut (as
seen to the left).
Note -- when removing the tires, the manual suggests a rubber mallet struck
on the inside of the tire to dislodge the tire. There is an extremely
tight tolerance on the inside of the nut assembly, so once the outside
nut is removed, I found that a balled-up fist struck where the tire is
exposed near the rear of the platform, several times, spinning the wheel,
will loosen the tire up and it will slip off easily.