How can I override strings?
The easiest method would be to implement a hook_form_alter inside a custom module that overrides the title 

https://api.drupal.org/api/drupal/core%21lib%21Drupal%21Core%21Form%21form.api.php/function/hook_form_alter/8.2.x

Some information how to create custom module.

https://www.drupal.org/docs/8/creating-custom-modules