Changing Posted Data

Hi, I have successfully been able to access the posted data by using:

function check_number_form_elements($cf7)
{
wpcf7_before_send_mail and $submission = WPCF7_Submission::get_instance();
if($submission) {
$posted_data = $submission->get_posted_data();

But I want to change some of the data before it is sent elsewhere.

I have tried this:
$posted_data[‘first-name’] = “James”;

return $cf7;

But this does not work.
Am I missing something?

Thanks for any help you can give.
Ian