how to modify Header and Footer

Hello Support Team,

1 - I build my theme on the desktop application of NP
2 - I exported the theme
3 - I installed the them in WP

everything is good. but:

A - I have to modify some little things in the footer
B - I have to add the multilanguage selector in the header

how can I do this modifications?
Can I do them via WP?
Or I have to modify the theme in NP application and the re-install the theme?

thanks in advance
denis