How do I take off her suit to readjust her undergarment and make the suit for better? I'm a newbie!
Pretty simple
1. Take the head off
2. You have to snap off the belt gently, you might have to reglue it on
3.Pull the boots off, the foot pegs are attached to the boots
Unzip the suit and carefully slide it off the figure
( do this gently and carefully, although the leather is flexible, you don't want use excessive force, you don't want to tear the material.
Once you get that suit off
The undergarment
1. Now there are certain ways to do this, some posters have different methods
2. What worked for me was detaching the legs from the ball joint socket, it's really simple to snap them off, just carefully pull them off again it is plastic so don't use a lot of force.
Here's an example I posted long ago using the Babydoll figure
3. Do the same with the arms, detach from the shoulder ball sockets ( very easy to snap arms and legs back on)
Once both the legs and arms are detached
4. Gently slide off the
undergarment
**Some people have done this without removing the arms and legs, but the undergarment is very thin, using this method IMO keeps the undergarment from tearing off, because it is made of delicate material and does tear if you try to force it off, so it's your choice what you decide. ** 
As far as refitting the undergarment, is your call
What I did
1. Once I slid the undershirt off and put it back on..
2. With the arms and legs still off carefully tug and pull the undershirt carefully up and away from the the leg's ball joint so it's not hanging over the ball socket in anyway
Basically all you want is too see is no white material hanging over the groove where the ball joint to the leg is located ( be patient with this) don't rush
3. Once you got this form fitted, continue to hold or pull back the white material tightly while gently snapping back the legs
Remember the trick is making sure you have the undergarment tugged or pulled back tightly or just enough so that it is not hanging over the groove to the ball joint, this also helps the get rid of the bunch or sagging in her butt.
4. Do the same with the arms, tug away and hold firmly the material from the arms while snapping back the arms.
Now be careful, since the ball joint is connected to the shoulders, what worked for me
Where the socket hole is from the shoulder, carefully pull back the undergarment and then snap the arms back into place
What you want to see is the the white material overlapping or resting over the shoulder or arms instead of inside the socket, this allows the arms to move more easily because the shirt isn't rubbing against the shoulder joint.
I know this sounds complicated, but once you actually use these methods you will see what I mean as far as everything fitting more tighter to the body, carefully eliminating the sagging and bunchy parts IMO.
After the arms and legs are snapped back on and making sure the shirt is fitted tight, just carefully slide the leather suit back on ( remove the hands btw)
Carefully zip her suit back up.
Before you snap the boots and belt back on, I suggest gently pulling down, tugging and holding the leather suit ( I guess you can call them the shorts of the leather suit) by the legs while snapping the boots back on.
This helps to eliminate the sagging around the crotch area people have been complaining about, it doesn't get rid of it, but it fits more tighter and secure IMO.
As for the belt, I found that there is a piece of tape on the inside of the belt, now it's up to you, I removed it, because I noticed it was much easier to fit her belt more tighter to the waist without it sliding .
IMO it makes the belt fit smoother on her waist
It also helps bring out her curves more on her hips as well while in the right pose IMO
Again it's up to you.
Good Luck
Looks great Kia, still waiting for my repaint to come back. When it does I can't wait to use it for this look. 
