how to change %%sep%% tag?

You can use this snippet:

function wploop_change_separator()
{
return '|';
}
add_filter('document_title_separator', 'wploop_change_separator');

Thanks

Are you using Yoast? They have their own setting for that, and some themes do too.
Can you add URL and WP admin please, and I’ll get one of the developers to check it out.