To purchase or upgrade a storage plan, log in to the ONE web application. For directions on how to do so, check out the Accessing your Billing and Account Information for ONE article
Once you have gained access to your billing page, navigate to the 'Account' tab and select the 'Change' button within the 'ONE plan' section of your account page. This should expand the section and show you several listings in either monthly or yearly format. You can toggle the subscription interval using the 'Show (monthly || yearly) plans'. Keep in mind that you are indicating the total amount of space that you would like in your account. You are not indicating the amount of space you want to add to your current plan.
If you have a promotional code you can enter it in the empty blue box beneath the plan listings then select 'Use Promo Code' to have the code applied to the listings and show you the promotional prices.
After you have made your selection, choose 'Next' to advance and select 'Pay via card'. Enter he appropriate information then select Next again, confirm your choices, and you're set!
If applicable, upgrades are pro-rated. If you are paying with a debit or credit card, then at the time of the upgrade you will be billed nothing. Then upon your renewal date you will be billed the pro-rated upgrade amount plus the normal renewal amount.
The upgrade should apply to your account shortly. You may need to restart the application to see the upgrade take effect. If you have any trouble during your storage space purchase such as missing plans or page-load failures please be sure that you do not have any browser extensions which may interfere with the page.