How to implement pagination in backward way eg. first – 20 – 19 – 18 – … – 3 – 2 – 1 – last

You could use wp_paginate_links(), pass it the 'type' => 'array' parameter in the argument array, and then array_reverse() the output.

But: if you’re doing this for SEO purposes, don’t bother. Your canonical URL should be your single-post view, not an archive index; so the index archive page numbers should have minimal SEO impact, no matter what order the page numbering.

EDIT

wp_paginate_links() Codex ref

EDIT 2

If you want to reverse the order of the posts themselves, you’ll either need to use a custom query, or modify the default Loop query, likely by changing the $order from DESC to ASC.

EDIT 3

If you just want to reverse the order of the posts themselves, then just change $order from DESC to ASC, and don’t do an array_reverse() on the wp_paginate_links() call.