You could measure your centre of gravity roughly, by lying on a rigid board on a pivot - a bit like a see-saw - and then moving the pivot around until the board (with you on it) is level. Your centre of gravity is then directly over the pivot. This assumes that your body is roughly symmetrical side to side.
Having your centre of gravity high up in your body (top heavy) would make you more likely to topple over. Having it lower down makes you less likely to topple.
I'm not sure how you would 'improve' your centre of gravity.
Are you using it in some mystical non-scientific sense?
The quickest way to lower your centre of gravity is to lower your arms.
At school, as a test of balance, we once had to lie on our backs on a 5cm wide plank (probably two inches wide in those days). The trick was to dangle your arms and legs to lower your centre of gravity.
They say that players like Geo Best and Wayne Rooney have a low centre of gravity which makes them more stable and have better ball control. At the other end of the scale we have that 6'7" tall England and Portsmouth player, although artistic looks like part of an octopus.
Bend your knees and spread legs apart slightly. See judo, fencing, ice skating and surfboarding for examples. Lower centre of gravity and broader base makes you harder to knock over and gives you better balance.