How to subtract value in datagridview in form1 and textbox from form2, and the result will change the value in datagridview in form 1 using C#

That depends on exactly what you are doing.
If you use Form2 in the same way you use a message box, or a FieOpenDialog but with an input then it's easy:
1) Add a property to Form2 which collects the value from the textbox and converts it to a number before returning it.
2) Then just get it:

Form2 f2 = new Form2(0);
if (f2.ShowDialog() == DialogResult.OK)
   {
   int value = f2.Value;
   ...
   }

If you want to do anything else, it gets more complex, but these will either help or confuse you:
Transferring information between two forms, Part 1: Parent to Child[^]
Transferring information between two forms, Part 2: Child to Parent[^]
Transferring information between two forms, Part 3: Child to Child[^]