:sigh:
Look at your code:
Form1 fm1 = new Form1();
fm1.UpdatingTextBox(value);
What does the
new
keyword do?
You know that: it creates a
new instance of the class - which you then update to reflect the new value. That won't be reflected in the the current display because that is a different instance of the Form from the one you just created.
You need to get the excel class to update the actual display - which its best handled by providing a "Changed" event which the form handles and updates itself.
See here:
Transferring information between two forms, Part 2: Child to Parent[
^] - think of your form as the Parent, and your excel class as the Child, and it should explain how to do it.