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.
The only remedy to having too little space on your device is to identify and remove unwanted data. You may find these help pages offer some useful information and insight:
How to check the storage on your iPhone, iPad and iPod touch – Apple Support
About storage on your device and in iCloud – Apple Support
If, when looking at your own data storage usage, you see that the “other” category is consuming an disproportionate amount of space on your iPad, this support page may be helpful to you:
Clear Other storage on your iPhone or iPad - Apple Support
I hope this information and guidance proves to be helpful in resolving the issue.