Translating WordPress (Theme Attitude)

Hello, I recently want to change some variables in the Attitude theme. For example:
echo '<div class="back-to-top"><a href="#branding">'.__( 'Back to Top', 'attitude' ).'</a></div>';

I understand __(‘Back to Top’, ‘attitude’ ) is a translation link to the .mo and .po file. However these are my questions:

1.) I know how to change the non-English translation from the po file, however how do you change the actual English text? Just by changing the code to __(‘new_Back to Top’, ‘attitude’ )? or do I have to change the all po files as well?

2.) If I insert more rows in the *.php file, do I have to alter the row number on the *.po file as well (if there are any translation)

3.) How can I change __(‘Back to Top’, ‘attitude’ ) to __(‘Back to Top’, ‘New_attitude’ )? Besides changing every translation requests like __(‘Back to Top’, ‘attitude’ ), do I have to do anything more?

Sorry it may seem very very basic to you. But I got stuck for a long time and after I learn all of these, I may be easier to pick up other ones myself. Many thanks. Gordon