When you upgrade to iCloud Drive, your existing iCloud storage plan doesn't change. You continue to have the same amount of storage as before. The only difference is that now you can store any type of file in iCloud Drive—including apps, documents, and desktop folders.
If you need more storage, you can upgrade your plan at any time from your iPhone, iPad, iPod touch, or Mac. On your iOS device: Go to Settings > [your name] > iCloud > Manage Storage or iCloud Storage. On your Mac: Open System Preferences > iCloud. Click Manage... next to your current storage plan.
You can also upgrade your storage plan from icloud.com:
Sign in at icloud.com with your Apple ID and password.
Click on Settings in the bottom left corner of the main page (looks like a gear).
In the main Settings page that appears, click Upgrade next to your current storage amount listed under STORAGE & BACKUP on the right side of the screen (or click Buy More Storage).