How can I read a specific line of text and replacing it with an option in the program

Hey,

So I’m wanting to read a specific line of text in a text document (line 10) and I’m wanting to replace what that text says. I have a combo box that has a list of options and I want to replace the text considering the option they chose. For example:

They select Russia in the combo box so line 10 in the text box changes to ” locale: “ru_RU””

Now how would I be able to do this? I’ve looked online but found nothing at all.

Any help would be great, thanks.