You have to be logged in with the current main user, in order to change the main user for a customer profile.
1. Select “My Account” from the main menu.
2. Select “Settings” from the left menu.
3. Scroll down until you find the category “Users”, and select “Show more” on the user you wish to set as main user.
4. Click on “Set as main user”
If you have further questions about your account, read our articles regarding the subject.
Or you could always contact support.