How do I buy more storage

Review the instructions in this guide published by Apple 👉 Upgrade to iCloud+ - Apple Support

The iPhone specific instructions are below:

1. Go to Settings, then tap your name. 
2. Tap iCloud, then tap Manage Account Storage or Manage Storage. Or scroll down and tap Upgrade to iCloud+.
3. Tap Buy More Storage or Change Storage Plan.
4. Choose a plan and follow the onscreen instructions.

Axel F.