send a value from one form to another form

If you look through the Windows Forms articles you will find many useful ones, including some that explain how to do what you are asking for here. There are many ways to do it.

The hard part is that the sending form must have a reference to the receiving form. There are many ways to do that. If form1 creates form2 then you have the reference you need.

Assuming that form1 has a reference to form2, create a method in form2 that takes the value as a parameter and that puts the value in the label. Make the method public. Then call that method (in form2) from form1.