wp-content/themes/Your theme name/header.php

Remove "|" from
If not, please tell me along with the theme name.

Please copy and paste the following at the bottom of functions.php.

function custom_title_sep ($sep) {
  $sep = '';
  return $sep;
}
add_filter ('document_title_separator', 'custom_title_sep');

If the title tag is directly written as asuchi0819 says,

If the old theme seems to have been output with wp_title (), delete the pipe as shown below

↓

Maybe

add_theme_support ('title-tag');

This is the type of theme that will be used in the title, or basic after WP4.4

This delimiter is filtered and defaults to '-'
If '|' appears, it should already be filtered
Priority with overriding priority

add_filter ('document_title_separator', function () {return '';}, 100);

---

Appendix

add_theme_support ('title-tag'); If document_title_separator doesn't work
You may have created your own title with pre_get_document_title

In this case, the title structure depends on the theme
How do you find the pre_get_document_title filter in the theme?

It's not a fundamental code clarification, but if you use the Lightning theme,

It can be changed from the plug-in of "VK All in One Expansion Unit" recommended to install at the same time.

First, from the "VK All in One Expansion Unit" menu
Check the "Rewrite title tag" checkbox in "ExUnit".

Next,
If you select "Top Page