Your photograph illustrates an issue with the screen backlight diffuser layer - that has partially delaminated within the screen assembly.
The only remedy for this fault is replacement of the iPad screen, or the iPad in its entirety.
You would be best advised to visit the Genius Bar at your local Apple Store or Apple Authorised Service Provider (AASP) where the technicians will be able to assess your iPad - and you can explore your available options:
Genius Bar Reservation and Apple Support Options - Apple
For practical purposes, in lieu of repairing and returning your own iPad, Apple usually exchange a qualifying iPad for a renewed/refurbished iPad of the same model and specification. The cost of the exchange is the same as would otherwise be charged for repair. Unless you have benefit of an active AppleCare plan, an out-of-warranty repair while expensive will be somewhat less than the cost of a replacement iPad.
As for out-of-warranty repair costs, Apple offers a fixed-price price repair. You can verify eligibility and cost for your region/country here:
iPad Repair & Service - Apple Support
If you have an active AppleCare+ plan, the cost of repair is reduced to a small excess/deductible fee.