Calculator help finish project

I´m a begginer at programing and right now im trying to make a calculator with help of a tutorials. I finished basic version of this calculator and now trying to improve it.
So there are my problems that i need to help with a lot:

  1. Button +/-
  2. HistoryText which should output the whole computation at the top after equals.
  3. Found a problem with changing operation while making a computation.

Here´s “my code” with a help of a tutorials.
Would love to if somebody finds time and help me with this thank you.