I assume that you do something like read each line from the file into a separate TextBox?
If so, then you need to loop through all the Textboxes in the panel (you can use its Controls collection to access them) and write each line back to a separate file.
But the question is: why are you doing that? Why not just set a single TextBox to
MultiLine=true
and use that for the whole file? Then it is just a case of:
myTextBox.Lines = File.ReadAllLines(path)
And
File.WriteAllLines(path, myTextBox.Lines)