Hello, @Sadikul Islam,
Can you please follow the instructions provided as a solution in this topic: https://www.8theme.com/topic/change-single-blog-title-heading-from-h2-to-h1/ I hope it will solve your issue and if not then please share your site FTP or Cpanel Login details in the private content area of this topic, so that I’ll further look into this for you on your site.
Regards 8Themes Team.