Thanks for showing interest in BizGrowth WordPress theme.
In BizGrowth free theme their is a problem in mobile version with menu. The menu overlays the homepage image slider, but with small css code we can solve this problem.
About three question:
1. To change the slider images automatically open jquery.nivo.slider.js file in js folder of theme directory and in line number 22 set
2. With PRO theme you can use all your previously created posts and pages. The PRO theme is build in user friendly theme option whereas the free theme is build in customizer.
The PRO theme contains lots of section in homepage to create beautiful and professional website. The PRO theme is very easy to setup and customize. We provide the documentation and user friendly support at any stage of website development. Please check the documentation
3. To change the Read more button in slider
Go to the Appearance -> Editor -> open header.php file and in line number 70 change the ‘Read more’ button text as you want.