I had to reassign a corrected share price and quantity for two stakeholders. I was able to modify the amount and number of shares by clicking the 3 vertical dots to the far right of their name when in the Securities Tab. You'll be given a box to explain why the change and even if you want to have the changes emailed to the shareholder. I believe the change/modification to the shareholder will be automatic if they've already signed their certificate. I really hope my information is not incorrect and helpful. Apologies if not.