There are a lot of wordpress theme does not display the edit link (edit button) in the article. To display the edit link in wordpress I use the function:

<?php edit_post_link( $link, $before, $after, $id ); ?>

In that:
$link: Edit text containing links that we want to show
$before: Edit link text ago (The default is no)
$after: Đoạn text sau edit link (The default is no)
$id: id article needs repair (default without)

Add a necessary thing is that we need check whether the user can edit not, if so, to show new edit the following link:

if ( current_user_can('edit_post', get_the_ID())

So complete code to display the edit link in wordpress:

<?php 
 	if ( current_user_can('edit_post', get_the_ID()) ) {
 	 	edit_post_link('edit', '', '');
 	}
?>