How to factory reset my deceased brother's iPad?

First:

How to request access to a deceased family member’s Apple Account - Apple Support

Then:

What to do before you sell, give away, or trade in your iPhone or iPad - Apple Support

Then you may set up this “like-new” (software wise) iPad using your own, unique Apple Account.

Condolences on your loss.

See:

How to factory reset your iPhone, iPad, or iPod touch - Apple Support

Note that if your brother set up Find My in the iPad, then the iPad will be in Activation Lock and not usable unless and until you can get into his Apple Account to release the lock. If you don't have his account information, see:

How to request access to a deceased family member’s Apple Account - Apple Support (CA)

Regards.