You need to search before asking. This question was answered 1000 times.
Here what you need to do:
Go to / qa-include / qa-lang-main.php
I think it's line 118,
Change this 'nav_qs' => 'Questions', with 'nav_qs' => 'Discussions,
/ qa-include / qa-lang-question.php
'answer_button' => 'answer', with 'answer_button' => 'reply',
I am not really sure if those are the correct files, but now you have an idea, and you can spend some time knowing your code.