I want to increase iPad Pro 12.9 2021 memory from 128GB to at least 256 after the week of buying it.

there are no way in the universe to increase local storage of ios devices, one can rent icloud storage but it's not the same thing

What's the difference between device storage and iCloud storage? - Apple Support

some countries offer a 14 days full refund of a product, if yours does you can return it and get your money back and add money to the pile and purchase an ipad pro with more storage.

No way to increase the internal storage of an iOS device.

You have 14 days (from the date of purchase) to return that iPad and ask for a refund.

or

you can try to sell it on your own, then add the money to buy a new model with your preferred capacity.

Regards

Giulio