I just started with Mathematica and coming from MathCAD because of its poor functionality in higher math.
Now I created a new Spreadsheet and was just trying around with some functions and variables.
Because I want to make some big calculations, I must have the possibility to sometimes changes some variables/functions at the start. Therefore i need the option to update my whole spreadsheet with these changes, without reentering all the later commands.
You can mark some cells as "Initialization cells". When you need to re-evaluate them with new values you just choose "Evaluate initialization" from the Evaluation menu. There is also the Dynamic mechanism. If you have
v = 2;
f[x_] := x^2;
and later change the value of v, the output of Dynamic will be updated immediately.