Manually update user_meta

Hello,
Could you please help me with manually updating wp_usermeta > wp_s2member_custom_fields meta? I’d like to add automatically some data from my standalone registration page.
I have the custom field defined in “open registration fields”, just need a snippet that will update given user wp_s2member_custom_fields with, lets say, country:england 🙂

Thanks!

https://wordpress.org/plugins/s2member/