Here is a quick tutorial on how to purchase additional iCloud storage.
Step 5:
Once in the Upgrade iCloud Storage menu, you can select the amount of iCloud storage you would like have on your account. The blue checkmark will appear next to the option you choose. You can then tap Buy in the top right corner.
Step 6:
When the Sign in to iTunes Store notification appears, enter your Apple ID password and press OK to complete the purchase.