You cannot upgrade the internal storage on an iPad after purchase
Buy an iPad with more built-in storage
Use iCloud storage for photos, files, backups, and app data
Go to Settings
Tap your name
Tap iCloud
Tap Manage Account Storage or Manage Storage
Tap Buy More Storage or Change Storage Plan
Choose a storage plan
Confirm the purchase with your Apple ID
Use external storage devices supported by your iPad model
Transfer files to a computer or cloud service to free up space
Delete unused apps, photos, videos, and downloads