Would updating my rooted Sony Xperia Z brick it? I updated my phone, rooted it and then realized the current version isn't the latest, so I would like to update it. Is there any chance my Xperia Z would brick? Or do I have to unroot it first?
No.

Instead you may lose your root access in turn, because some system settings go back to default when updating a device, which will probably include your superuser.

Your personal data and installed apps will not be affected.