SpiderOak.com Support's dashboard

This is a option in your application. Please click account (top right corner) and then you will see a the "edit" button in the shareid section. Change it to what you desire, and it will be modified.

Reminder, you can only have 1 shareid, but many share rooms. If you have existing share rooms with that id, you will need to provide the new link to the people you are sharing with.