It isn’t possible to upgrade or extend the internal storage of your iPad. The internal storage configuration of your iPad is fixed during manufacture - and will be the same for the life of the iPad.
About storage on your device and in iCloud – Apple Support
By modern standards, whilst some users manage with only modest amounts of internal storage, 64GB isn’t that much. If storing photos, documents and download, it is quite easy to exhaust this amount of storage. Your options are limited - in that you can either better manage the storage that you already have, or replace the iPad with a model that has sufficient storage for your needs.
How to check the storage on your iPhone, iPad and iPod touch – Apple Support
Manage your iCloud storage – Apple Support
Clear Other storage on your iPhone or iPad - Apple Support
If your iPad is a recent purchase directly from Apple, you have 14-days in which to unconditionally return the iPad for exchange or refund. After 14-days, you have the option of a trade-in:
https://www.apple.com/shop/trade-in
Should you consider trade-in, you need to be aware that its value will be much lower than you paid for the iPad. Many find that selling their iPad privately achieves a far better value. If you sell or trade-in your iPad, you should properly prepare it before parting with it - as this will ensure that your data and the Activation Lock is properly removed:
What to do before you sell, give away or trade in your iPhone, iPad or iPod touch – Apple Support
I hope this information and guidance proves to be helpful in outlining your available options.
