Hi Judy5F88,
Thank you for reaching out and sorry to hear you are facing trouble using Adobe XD. Basically you cannot update/create your private/public link to link with password after creation of link and you can only create link with password before creation of the link. Now if you need to create link with password means you need to create new link.
Please go through the below attached helpx link to know more on link creation with password:https://helpx.adobe.com/xd/help/create-shareable-links.html and also please see the attached video for reference.
Please let us know if you need more assistance on this. We'd be happy to help.
Thanks,
Chethan G