As the instant value of any variable can be explicitely changed, there is a possibility to create a function of this variable - the iteration rule.

  The simplest example:
Create a variable c=0.
Click the button "VALUES"  . The window to display instant values of variables and functions opens.
Let c has to be incremented by one in each next step.
Assign Editor with the expression c+1 (the iteration rule).
Select the symbol c. Now it is possible to push the button "EDITOR VALUE -> VARIABLE VALUE"  to assign the value of the variable with the value of the function written in Editor.
The click on the button the incrementation of c by 1.

  Try to change the iteration rule:
Write new iteration rule into Editor: c-1.
Now each click on the button causes the decrementation of c by 1.
But the rule can be commonly any function of the variable.
Try 2*c+1 or c+c/5 or sin(c^2) for example.

  The automatic iteration:
There is no need to perform the iterating steps by hand, the value of the variable can be "locked" to the value of Editor. There is the button "LOCK"  for this purpose.
Create a variable c=1.
Assign Editor with the expression c*1.01
Select the symbol c and push the button "LOCK" and observe the value of c in instant values' window. It is increasing as the iterations are performed repeatedly and automatically. Try to change the iteration rule in Editor during the iterations and observe the value of c!