Believe it or not, you're not the first one to do this. Hence why there's a help guide to walk you through it. When a user forgets to add an approver onto an approval request, you can go to the In Progress list and select your project. Then scroll down to the Approvals section. Click the Blue Square button to view more information on your existing approval requests.
Click the button on the approval you'd like to amend. Then click Edit Approvers.
Add or remove your approvers as necessary, or for more information, check out this guide on editing approvers. Then click Done!