Override Yoast SEO existing title or meta template variable

functions.php

function filter_wpseo_replacements( $replacements ) {
    if( isset( $replacements['%%currentmonth%%'] ) ){
        $replacements['%%currentmonth%%'] = 'Sept';
    }
    return $replacements;
};
// Add filter
add_filter( 'wpseo_replacements', 'filter_wpseo_replacements', 10, 1 );