First, how to make changeable controllers, so that anyone can change controllers of their choice. Like any player can redefine keys according to their needs…
Second, how to make my character to walk on the ground, jump and fly too. I know how to make the character move on the ground and jump but what i need to know is how to make him fly too with the previous functionalities. I mean that he should be able to do all three walk on the ground, jump and fly in the air too. So that the player do what ever he wants(walk,fly or jump), when he want!!!
Please don’t shout (at least it seems that) 
Make changeable controls is a little hard to achieve, but I think it’s possible.
Instead use the “A key is pressed” condition, you’ll need “A key is pressed (text expression)”, you’ll need some string variables too (one variable for each key).
Finally, the hardest part is to write “code” so you can set the variables string in function of the pressed key (when changing controls); it’s hard because there is not such function (I think it hasn’t been added yet) and the “Text entry” doesn’t support strigs as “Space”, “left”, “right”, etc. I think you can do it but with a lot of repeated code (I am not fan of this) 
A nice workaround would be to change between a platformer character (can walk and jump) and a non-platformer character (when fly), just de/activate the platformer automatism when fly/walk and jump. Of course you’ll have to set extra code to control the player when fly (with non-platformer automatism actions).
Another way to do it is, when fly, set falling speed = 0 or something like that, again you’ll need extra code to control the player in that circumstances 