In Sifter, there is no way to simply change your primary company, you have to change the name of the company then move everyone around manually. You must be an Account Holder or Administrator in order to do this.
You can change the name of an organization by going to the People & Organizations page and selecting the People tab. From there, click on the name of the organization you want to change (the primary organization will be the first at the top of the list), enter the new name, and click "Save Changes".
To move people from one company to another you’ll have to go to each user’s profile from the People tab. From there go the the User Organization page in the sidebar menu and choose the correct organization from the drop down menu.