How to change sign-up fee text in WooCommerce Subscriptions

Try this:

function change_subscription_product_string( $subscription_string, $product, $include )
{
if( $include['sign_up_fee'] ){
$subscription_string = str_replace('sign-up fee', 'your text', $subscription_string);
}
return $subscription_string;
}
add_filter( 'woocommerce_subscriptions_product_price_string', 'change_subscription_product_string', 10, 3 );