FTP help with child theme

Hi all.

I was using a free trial version of ClassicFTP when I first created my blog, and created a child theme straight away using this ftp provider.

That trial has now run out, and I am now going to use a paid version from a different provider (not sure who yet, recommendations welcome).

I wanted to add a Pinterest code to verify my website and have found out that I need to change the header.php file in my child theme.

2 questions:

1) Would I need to open up the header.php file from within the FTP in order for WordPress to understand that I have made a change? Or can I do it just from accessing the file in my folders on my Mac, and then upload it via FTP?

2) Since I am changing FTP providers, please could someone advise if I need to go through the whole child theme creation again? Or is it just a case of me getting a new provider, and moving the child theme onto it, and then reselecting the child theme from within WordPress?

Thank you!