Use the Profile button in the upper left corner of the page to manage your account settings:
There are three tabs: Main settings, API key and Invoice details.
On the Main settings tab you can edit your personal details (First and Last name, Skype account), find your referral link and change your Timezone. You can also set a new password there.
On the Invoice details page you can enter your billing address and your VAT id. Please mind that additional VAT taxes will be applied if the TAX id is not provided.