When you upgrade to a higher plan, you do not purchase additional space, you purchase an entirely new plan. Rather than selecting the amount of space you want to add, you should select the total amount of storage you would like to have in your plan.
If you have a 1 TB plan, then you would select the next size plan, 5 TB.
For instructions on how to access your account and purchase a plan, see Purchase or Upgrade Storage for ONE accounts