Unable to find theme editor for twenty nineteen

Do you see the Customize option under the Appearance menu in your site’s Dashboard?

Yeah i can see that. But i want to edit header.php file of the theme.

Make sure you haven’t disabled that editor via a plugin or the modification detailed at https://codex.www.ads-software.com/Editing_wp-config.php#Disable_the_Plugin_and_Theme_Editor

@manuraj09, it’s not advisable to edit the theme or plugin files using that built-in editor. Alwasy use 3rd party editors to edit core Plugin and/or theme files when required.