where do you click change storage plan?

This is in your settings in iCloud, manage storage (see below for specific steps to help you out) 🙂

Upgrade your iCloud storage from any device

Whether you’re on an iPhone, iPad, iPod touch, Mac, or PC, upgrading your iCloud storage is simple. Choose your device below and upgrade in just a few quick steps.

On your iPhone, iPad, or iPod touch

1. Go to Settings > [your name] > iCloud > Manage Storage or iCloud Storage. If you're using iOS 10.2 or earlier, go to Settings > iCloud > Storage.
2. Tap Buy More Storage or Change Storage Plan.
3. Choose a plan.
4. Tap Buy and follow the onscreen instructions.

On your Mac

1. Choose Apple menu > System Preferences, then click iCloud.
2. Click Manage in the lower-right corner.
3. Click Change Storage Plan or Buy More Storage.
4. Choose a plan.
5. Click Next and enter your Apple ID password.

On your Windows PC

1. Open iCloud for Windows.
2. Click Storage.
3. Click Change Storage Plan.
4. Choose a plan, then click Next.
5. Enter your Apple ID password, then click Buy.