where i can change header or footer or main page contents

Where I can change contents of header, footer or main contents?

Make a copy for your template of header.tpl, footer.tpl, or main_page.tpl

https://www.zen-cart.com/content.php?289-what-is-template_default
https://www.zen-cart.com/content.php?78-how-do-i-set-up-the-template-overrides
https://www.zen-cart.com/content.php?80-how-do-i-find-out-what-files-to-edit

tpl_header.php, tpl_footer.php, and tpl_main_page.php can be found in the common directory of the applicable template folder. If the file doesn't exist in the active template folder, then the one in the template_default is used: includes/templates/YOUT_TEMPLATE/common

The main_page does somewhat consist of more than the one file above, as there is also define_main_page.php which is in your includes/languages directory. This is content that is modifiable in the admin unde tools->define pages... It also uses a template override system, but the file must first be copied using ftp into the active template directory.